Rename the persistent agent integration to Sybil Agent

Use SYBIL_AGENT_BASE_URL and SYBIL_AGENT_API_KEY with the renamed sybil-agent
service. Update errors, API documentation, and test fixtures while retaining the
existing room protocol and browser storage keys for compatibility.

Validation: 44 server tests, 51 web tests, and both production builds.

web/tests/agent-room.test.mjs

@@ -9,7 +9,7 @@ import { createAgentOptimisticOutbox } from "../src/lib/agent-optimistic-outbox.
 
 const message = (id, role = "assistant", extra = {}) => ({
   id: `entry:${id}`, createdAt: `2026-10-14T10:00:${String(id % 60).padStart(2, "0")}.000Z`,
-  role, content: `message ${id}`, name: role === "assistant" ? "Pi" : "buzzert",
+  role, content: `message ${id}`, name: role === "assistant" ? "Sybil Agent" : "buzzert",
   metadata: { kind: "message", entryId: id, status: "complete" }, ...extra,
 });
 const page = (cursor, messages = [], hasMore = false) => ({
@@ -183,7 +183,7 @@ test("malformed successful receipts keep optimistic messages retryable under the
   assert.equal(row.metadata.status, "failed");
   assert.equal(box.snapshot()[0].seenOnServer, false);
   const valid = { id: 1, status: "queued", messageId: row.id };
-  for (const malformed of [null, { error: "Invalid Pi response" }, { ...valid, id: 0 }, { ...valid, id: 1.5 },
+  for (const malformed of [null, { error: "Invalid Sybil Agent response" }, { ...valid, id: 0 }, { ...valid, id: 1.5 },
     { ...valid, id: Number.MAX_SAFE_INTEGER + 1 }, { ...valid, status: "unknown" }, { ...valid, status: ["queued"] },
     { ...valid, messageId: "inbound:sybil:other" }]) {
     response = malformed;