feat(agent): add authenticated durable uploads to backend and web

docs/api/rest.md

@@ -606,7 +606,7 @@ messages.
 ### `POST /v1/agent/messages`
 
 Body: `{ "content": "Check the oven", "requestId": "client-generated-uuid" }`.
-Content must contain non-whitespace text and is limited to 40,000 characters;
+Content is limited to 40,000 characters; it must contain non-whitespace text unless attachment IDs are included;
 `requestId` is required (1–256 characters). Preserve the ID when retrying uncertain
 network results. The proxy fixes `channel: "sybil"`, `whenBusy: "followUp"`, and
 `wait: false`.
@@ -614,9 +614,9 @@ network results. The proxy fixes `channel: "sybil"`, `whenBusy: "followUp"`, and
 Returns the agent's `202 Accepted` receipt, including `id` (submission ID), `status`, and
 `messageId` such as `inbound:sybil:client-generated-uuid`. This is admission, not
 completion: watch the room for answers, including unsolicited later replies.
-Multiple requests can be accepted while the agent works. Attachments are not yet supported.
+Multiple requests can be accepted while the agent works. Optional `attachments: [{"id":"att_…"}]` accepts uploaded files as specified below; no client paths or inline file bytes are accepted.
 The proxy verifies successful receipts contain a positive submission ID, a known
-status (`queued`, `placed`, `done`, or `unanswered`), and the expected `messageId`.
+status (`queued`, `placed`, `done`, or `unanswered`), and the expected `messageId`. Attached sends also require matching ordered `attachmentIds` in the receipt.
 Invalid receipts return 502; delivery remains uncertain, so retry using the original
 `requestId` to avoid creating a duplicate message.
 
@@ -632,3 +632,55 @@ after showing the corresponding history; poll it to reconcile unread indicators
 across clients. Agent REST responses use `Cache-Control: no-store`.
 
 See the persistent Agent WebSocket section in `streaming-chat.md` for live updates.
+
+### Agent attachments (v1)
+
+`POST /v1/agent/attachments`: multipart/form-data with exactly one `file` part (original UTF-8 filename in Content-Disposition) and one `uploadId` field (client-generated UUID, stable across retries). Either part order is accepted. Request limit 8 MiB + 64 KiB. 201 new / 200 identical retry:
+
+```json
+{"attachment":{"id":"att_12345678-1234-4234-8234-123456789abc","kind":"text","filename":"notes.txt","mimeType":"text/plain","sizeBytes":12,"sha256":"0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef","workspacePath":"/data/workspace/uploads/att_12345678-1234-4234-8234-123456789abc/notes.txt"}}
+```
+
+`workspacePath` is OPTIONAL for clients, canonical absolute runtime path when supplied. Never a host path, URL, or request parameter. Descriptors must retain filename/mimeType/sizeBytes/sha256. `kind` is `image`, `text`, or `file`. IDs are opaque. UUID retries must match normalized filename, effective MIME type and bytes, otherwise 409 `idempotency_conflict`. Filename is NFC normalized, 1–255 UTF-8 bytes, no slash/backslash/control characters, not `.`/`..`. A leading dot becomes `_` in the on-disk basename only; display filename remains unchanged.
+
+`POST /v1/agent/messages`:
+
+```json
+{"content":"Read these notes","requestId":"client-stable-id","attachments":[{"id":"att_12345678-1234-4234-8234-123456789abc"}]}
+```
+
+`content` is required, <=40,000 characters; empty/whitespace is allowed with attachments. `requestId` is 1–256 characters and must survive retries. Omit attachments for text-only requests. At most 8 distinct references, 32 MiB total original bytes. References contain ONLY `id`; filenames, paths, URLs, base64 and forged workspacePath are rejected. Root delivery uses channel sybil, followUp and wait=false. 202 receipt:
+
+```json
+{"id":123,"status":"queued","messageId":"inbound:sybil:client-stable-id","attachmentIds":["att_12345678-1234-4234-8234-123456789abc"]}
+```
+
+Receipt status also permits placed/done/unanswered. `attachmentIds` is present for attached input, in request order, and MUST be checked before treating delivery as acknowledged. Caption + ordered IDs are durably bound to requestId; conflicting replay 409. Native admission atomically stores the input payload, binding, quota charge and worker eligibility; a competing text-only input cannot falsely acknowledge attachment IDs. Network errors may be retried with the same request/upload IDs. Never auto-create a fresh ID on ambiguous failure.
+
+History, replay and queued activity use the existing public message schema with `metadata.agentAttachments` containing these descriptors. `content` is the original caption, or `Attached: notes.txt.` for a file-only message. No model-only manifest, text preview or base64 is inserted into public message content. Ordinary chat's `metadata.attachments` schema is unchanged. Raw tool rows stay in the protocol; client transcript filters stay unchanged.
+
+`GET /v1/agent/attachments/:id/content`: authenticated original bytes, Content-Length, Content-Disposition attachment (safe ASCII filename and UTF-8 filename*), Cache-Control private/no-store and nosniff. No URLs containing tokens, public static mount, redirects or inline serving. Browser previews use authenticated fetch + revocable blob URL. Native downloads hold one of four slots through response finish/close/error, with a 30-second outbound timeout and disconnect cleanup. Raster types retain their validated MIME type; text downloads use text/plain; other files application/octet-stream. Clients can display/copy workspacePath as text.
+
+### Attachment capability and errors
+
+Optional `GET /v1/agent` field:
+
+```json
+{"capabilities":{"attachments":{"version":1,"maxCount":8,"maxTotalBytes":33554432,"maxImageBytes":6291456,"maxFileBytes":8388608,"imageMimeTypes":["image/png","image/jpeg","image/gif","image/webp"],"maxImagePixels":50000000,"maxImageDimension":16384,"quotaBytes":1073741824,"usedBytes":0,"remainingBytes":1073741824,"maxConcurrentUploads":2,"maxConcurrentDecodes":1}}}
+```
+
+Quota values are a snapshot, not a reservation. Absence disables attachment selection and sending. Backend rejects attachments against an old/unsafe upstream rather than allowing dropped references. Sybil also requires ADMIN_TOKEN for attachments; text-only open development behavior remains compatible.
+
+Errors: `{ "error": true, "message": "...", "statusCode": 507, "code": "attachment_quota_exceeded", "quotaBytes": 1073741824, "remainingBytes": 1200, "requiredBytes": 20000 }`. Quota numbers are included for 507; no host filesystem information. 400 invalid_upload/invalid_attachment; 401 authentication; 404 attachment_not_found; 409 idempotency_conflict; 413 attachment_limit_exceeded; 415 unsupported_media_type; 422 invalid_image/image_model_unsupported; 429 attachments_busy with Retry-After: 2; 503 attachments_unavailable; 507 attachment_quota_exceeded; 502 unavailable/invalid upstream at proxy. Clients show message and retain retry identity. Remove from composer does not delete completed durable uploads.
+
+### Runtime access and release requirements
+
+The runtime resolves IDs to persistent `WORKSPACE_DIR/uploads/<id>/<safe-original-basename>` files (default `/data/workspace/uploads/...`), backed by the existing `/data` mount. Images are native model image blocks, never remote links: supported PNG/JPEG/GIF/WebP originals are at most 6 MiB and are prepared as a bounded static first frame (orientation corrected, at most 2000×2000 and 4.5 MiB base64, further limited by the actual model). Reject unsupported image models; no arbitrary document/file provider blocks. Other files are at most 8 MiB and are available through actual workspace read/bash tools. Valid recognized UTF-8 text receives a bounded preview of 16,000 Unicode characters per file / 64,000 per input. PDF/archive/binary files are paths plus metadata. Original hash, size and confinement are validated during resolution. Root and workers share the same workspace; explicit `subagent` spawn/send `attachmentIds` persist in durable Reporter tasks and use the same resolver. Preparation saturation retries through a durable checkpoint after two seconds using the same Reporter request ID. Permanent admission failures, including quota exhaustion, reach the parent through the durable worker-report path. Uploads never auto-execute.
+
+Global `SYBIL_AGENT_ATTACHMENT_QUOTA_BYTES` defaults to 1 GiB. Concurrent reservations cover originals, static renditions, staging, metadata and conservative attachment input copies. Quota is conservative accounting, not free disk space. No complete upload expiry or delete endpoint is enabled. Removed composer files stay stored; crashed/uncertain reservations stay charged across restart until deliberate operator reconciliation. Ordinary authorized workspace tool writes and unrelated conversation history are outside attachment-ingestion accounting. Runtime uploads default to two concurrently (`SYBIL_AGENT_ATTACHMENT_UPLOADS`, max8), image decodes one (`SYBIL_AGENT_ATTACHMENT_DECODES`, max4); bounded input preparation and proxy transfers return429 when saturated. Disconnected proxy transfers cancel upstream work and release slots.
+
+Release requires native `SYBIL_AGENT_AUTH_MODE=token` with nonempty `SYBIL_AGENT_API_KEY`, plus backend `ADMIN_TOKEN` and matching server-side `SYBIL_AGENT_API_KEY` / `SYBIL_AGENT_BASE_URL`. Native open mode cannot advertise/store/download attachments. A runtime containing attachment records/files refuses open startup; raw/model APIs and sockets are protected in token mode. An open Sybil proxy also refuses access to an attachment-capable native runtime, rather than exposing its history through a private server credential. Explicit text-only open development remains supported with an open/legacy runtime lacking attachment capability. Native keys never enter browser/iOS payloads. No insecure development opt-in is enabled.
+
+The runtime source is built into its container: rebuild it with pinned busboy/sharp dependencies for the target Node/Linux platform before release. The deployed open auth mode must be changed separately; no deployed config or mount is modified by this implementation. Backend and web must be upgraded with runtime capability/auth configured before clients advertise uploads.
+
+Browser drafts retain upload IDs and completed descriptors across pane changes and reload. Incomplete uploads require reselection of the original filename/type/size and, once network delivery has begun, matching SHA-256 bytes; retries reuse uploadId. Sending persists the attachment-bearing outbox before clearing the draft. Message confirmation requires the same ordered attachment IDs in receipts and canonical echoes. Mismatched echoes must retain local attachments and delivery errors; they are not acknowledgement of a file submission.