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.

server/prisma/schema.prisma

@@ -216,8 +216,8 @@ model ProjectItem {
   @@index([searchId])
 }
 
-// Shared personal read position for the external persistent Pi room.
-// Messages remain in Pi's durable transcript, not duplicated in Sybil Chat.
+// Shared personal read position for the external persistent Sybil Agent room.
+// Messages remain in the agent's durable transcript, not duplicated in Sybil Chat.
 model AgentReadState {
   id        String @id
   cursor    String @default("0")