feat(chat): project-scoped chat — mcpctl chat --project <name>

Chat directly with a Project (no Agent needed): its Prompts become the system
context, its MCP-server tools are callable, its llmProvider/llmModel drive the
LLM, and (opt-in) the model can read secret values. History is saved inside the
project, attributed per user, resumable, and deletable (RBAC-permitting) — "use
it like Claude, scoped to the project".

Backend (reuses the agent-chat orchestrator):
- ChatThread is now agent-XOR-project (schema + migration + CHECK constraint);
  new listThreadsByProject / deleteThread on the repo.
- ChatService: prepareProjectContext (project prompt + Prompts by priority,
  llm from llmProvider with llmModel override, project tools), shared
  runChatLoop/runChatStreamLoop, project thread CRUD with owner enforcement
  (404-not-403 on foreign threads), admin-override delete.
- Gated get_secret virtual tool: offered only with --allow-secrets AND the
  caller's view:secrets; resolves via SecretService, never routes to a server.
- routes/project-chat.ts (chat SSE+non-stream, threads create/list/delete);
  RBAC run:projects:<name>.

CLI:
- `mcpctl chat --project <name>` (+ --allow-secrets), one-shot/REPL/resume.
- REPL /threads, /resume <id>, /delete <id>; project-aware header + /tools.
- `mcpctl get threads --project <name>`, `mcpctl delete thread <id> --project`.
- completions regenerated (--project completes project names).

Tests: 8 new project-chat unit tests; full mcpd (945) + CLI (508) green;
schema validated against Postgres. Docs: docs/chat.md "Project chat" section.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/db/prisma/schema.prisma

@@ -336,6 +336,7 @@ model Project {
   skills         Skill[]
   mcpTokens      McpToken[]
   agents         Agent[]
+  chatThreads    ChatThread[]
 
   @@index([name])
   @@index([ownerId])
@@ -751,18 +752,24 @@ model PersonalityPrompt {
 
 model ChatThread {
   id         String   @id @default(cuid())
-  agentId    String
+  // A thread is scoped to an Agent XOR a Project (enforced by a CHECK
+  // constraint in the migration + the service layer). Project-scoped threads
+  // back `mcpctl chat --project <name>`.
+  agentId    String?
+  projectId  String?
   ownerId    String
   title      String   @default("")
   lastTurnAt DateTime @default(now())
   createdAt  DateTime @default(now())
   updatedAt  DateTime @updatedAt
 
-  agent    Agent         @relation(fields: [agentId], references: [id], onDelete: Cascade)
+  agent    Agent?        @relation(fields: [agentId], references: [id], onDelete: Cascade)
+  project  Project?      @relation(fields: [projectId], references: [id], onDelete: Cascade)
   owner    User          @relation(fields: [ownerId], references: [id], onDelete: Cascade)
   messages ChatMessage[]
 
   @@index([agentId, lastTurnAt(sort: Desc)])
+  @@index([projectId, lastTurnAt(sort: Desc)])
   @@index([ownerId])
 }