Wire the chat interface in ~/src/mosaic-stack to the backend API.

## Context
The chat page (/chat) exists with full UI components but is NOT connected to the backend. Everything is stubbed. The old jarvis codebase had a working implementation — use it as the reference.

## Architecture Decision
Use `/api/chat/stream` (chat-proxy → OpenClaw) for LLM responses.
But ALSO wire conversation persistence using the existing brain + conversation-archive endpoints.

## Backend Endpoints Available (all authenticated, cookie-based auth):

### Chat/LLM:
- POST /api/chat/stream — SSE stream proxied from OpenClaw
  - Request body: { messages: [{role, content}], model?: string }
  - Response: SSE stream (pass-through from OpenClaw — handle both OpenAI format and custom format)
  - OpenAI streaming format: data: {"choices":[{"delta":{"content":"token"},"finish_reason":null}]}\n\n
  - Final: data: [DONE]\n\n OR data: {"choices":[{"finish_reason":"stop"}]}\n\n
  - Error: event: error\ndata: {"error":"message"}\n\n

### Conversation Archive (for persistence):
- GET /api/conversation-archives — list user's archived conversations
- POST /api/conversation-archives — create/save a conversation
  - Body: { title: string, summary?: string, messages: [{role, content, timestamp?}], metadata?: object }
- GET /api/conversation-archives/:id — get specific conversation
- DELETE /api/conversation-archives/:id — delete

### Brain (for context):
- POST /api/brain/query — query tasks, projects, events
- GET /api/brain/context — get summary context

## What to Change

### 1. apps/web/src/lib/api/chat.ts
Rewrite `streamChatMessage` to:
- Call `POST /api/chat/stream` (not /api/llm/chat)
- Use credentials: "include" (cookie auth)
- Add X-Workspace-Id header from workspace context
- Handle OpenAI SSE format: parse `data.choices[0].delta.content` for tokens
- Also handle `data: [DONE]` as completion
- Keep the same callback signature: onChunk(token), onComplete(), onError(err)

Add new functions:
- `saveConversation(title, messages)` → POST /api/conversation-archives
- `listConversations()` → GET /api/conversation-archives
- `getConversation(id)` → GET /api/conversation-archives/:id
- `deleteConversation(id)` → DELETE /api/conversation-archives/:id

### 2. apps/web/src/hooks/useChat.ts
- Fix `loadConversation`: load from conversation-archives API instead of ideas/brain
- Fix `sendMessage`: uses streamChatMessage correctly with the messages array
- Add `saveCurrentConversation()`: save to conversation-archives after each assistant reply
- Wire up auto-save after each successful assistant response

### 3. apps/web/src/components/chat/ConversationSidebar.tsx
Check if it loads from conversations API — if it's stubbed or uses wrong endpoint, fix to use `listConversations()`.

## Key Reference: Old Jarvis Implementation
The old jarvis at ~/src/jarvis-old/apps/web/src had:
- components/Chat.tsx (1616 lines — working SSE stream handler)
- lib/api.ts — working API client
- hooks/useConversations.ts — conversation list management

Read these for reference on SSE parsing, error handling, conversation state management patterns.

## Constraints
- Keep auth as cookie-based (credentials: "include") — NOT Bearer token for web client
- The X-Workspace-Id header must be included on all API calls
- Use useWorkspaceId() hook to get workspace ID
- Keep TypeScript strict — no `any` unless unavoidable
- Match existing chat.ts style (callbacks not promises for streaming)

## Process
1. git checkout main && git pull --ff-only origin main
2. git checkout -b feat/wire-chat-interface
3. Read the existing files: apps/web/src/lib/api/chat.ts, apps/web/src/hooks/useChat.ts, apps/web/src/components/chat/*.tsx
4. Read reference: ~/src/jarvis-old/apps/web/src/components/Chat.tsx (lines 400-550 for SSE handling)
5. Implement the changes
6. Run: pnpm turbo lint typecheck --filter=@mosaic/web
7. Commit --no-verify: "feat(web): wire chat interface to /api/chat/stream and conversation-archives"
8. Push and PR: ~/.config/mosaic/tools/git/pr-create.sh -t "feat(web): wire chat interface to backend" -b "Wires the stubbed chat page to /api/chat/stream (OpenClaw proxy) for LLM responses and /api/conversation-archives for persistence. References old jarvis implementation for SSE parsing patterns."

When done:
openclaw system event --text "Done: chat wiring PR ready" --mode now