…E tests

- Add GET /api/v2/chats endpoint to list user's chats
- Add ListChatsByOwner database query with proper authorization
- Build dynamic system prompt with workspace context when chat is bound to a workspace
- Default chat metadata to {} when empty to fix postgres JSON error
- Add E2E test TestChats_WorkspaceBash_E2E validating full chat flow:
  - Chat creation with workspace binding
  - LLM tool invocation (coder_workspace_bash)
  - Agent command execution via tailnet
  - Tool result propagation
- Add integration test TestChats_AnthropicIntegration for basic LLM connectivity
- Regenerate API docs and SDK types

This commit adds frontend support for the control-plane agent chats feature:

Frontend:
- ChatsPage: List chats with create dialog (model defaults to claude-opus-4-5)
- ChatPage: Real-time chat interface with WebSocket message streaming
- API client methods for chats CRUD and messaging
- React Query hooks for data fetching
- Navbar integration with Chats link

E2E Tests (site/e2e/tests/chats.spec.ts):
- create a chat and navigate to chat detail
- send a message and receive a response (uses claude-haiku-4-5, skips if no ANTHROPIC_API_KEY)
- chat list shows created chats

Bug Fixes:
- Fix panic in coderd/chats/hub.go: prevent double-close of subscription
  channel by wrapping close logic with sync.Once

Other:
- scripts/develop.sh: improve server startup detection

Redesign ChatsPage for immediate interaction:
- Chat textarea at top, auto-focused on page load
- Type message and press Enter to create chat and send
- Recent chats list below the input
- Click recent chat to navigate to detail view

ChatPage updates:
- Handle ?message= query param for initial message from ChatsPage
- Auto-send initial message when navigating with query param

E2E tests updated:
- chats page has input textarea ready for immediate use
- typing a message and pressing enter creates chat and sends
- recent chats are shown below the input
- clicking a recent chat navigates to chat detail

All 5 tests pass.