Chat panel: replace clipboard with real MCP-proxy to a model backend

[Astralchemist]

The current chat panel (packages/web/src/app/Chat.tsx) is compose-only — it builds a prompt with the selected anchor prefixed as `[About ]` and copies it to the clipboard. Per phase3plan.md §4.6, Phase 2 was supposed to be MCP-proxy to the user's running Claude Code session, but on closer reading that's actually awkward (Claude Code is an MCP client, not a server we can connect to from the browser).

Realistic next-step shapes:

1. POST /api/chat/send that streams a response from a configured model backend (Claude API / OpenRouter / OpenAI compatible), with the rig MCP tools available as function-call tools (sub-agent style — heavy tools allowed because the chat session is not the user's main agent session). Plays well with the existing OpenRouter target stub (OpenRouter target: confirm intended consumer #1).
2. Headless Claude Code spawn — `rig serve --web` could optionally fork a Claude Code session with the rig MCP server already wired, then websocket-bridge prompts to it.
3. Defer entirely to Phase 3 co-cartographer when write tools land — the propose/accept loop needs a chat channel anyway.

Recommend (1) once the model-backend abstraction exists. Until then the clipboard flow ships and gets out of the way.