AD-402: Add chat workspace API

[alph-cmky]

Summary

• Adds in-memory chat workspace service for channel creation/listing.
• Adds message creation/listing with auto-created threads.
• Creates pending task refs when messages mention valid agents.
• Rejects messages that mention unknown agents before storing them.
• Adds thin route handlers for channel and message operations.
• Marks AD-402 complete in the development backlog.

Stack

• Stacked on AD-401: Define chat and task schemas #37 (feat/ad-401-chat-task-schemas).

Verification

• TDD red check: pnpm test apps/server/src/chat/chat-service.test.ts apps/server/src/chat/chat-routes.test.ts initially failed because chat-service.js and chat-routes.js did not exist yet.
• pnpm test apps/server/src/chat/chat-service.test.ts apps/server/src/chat/chat-routes.test.ts
• pnpm lint && pnpm format && pnpm typecheck && pnpm test && pnpm build

Closes #13