Document how to enable MCP tools in the production image

## Task

Known limitation: MCP servers need `pip install -e ".[mcp]"` in the production image when `MCP_SERVERS_CONFIG` is set. That is easy to miss during a VPS deploy.

## Files

- `docs/FAQ.md`
- `docs/CONFIGURATION.md` (`MCP_SERVERS_CONFIG`)
- `docs/architecture/mcp-tool-router.md` or `docs/architecture/mcp-runtime-discovery.md`
- `docs/release/known-limitations.md` (already mentions it; link the FAQ)

## Acceptance criteria

- [ ] FAQ answers “How do I turn on MCP tools in production?”
- [ ] Points at the extra, env var, and that the default prod image may not include it
- [ ] No behavior change

Read docs/FAQ.md and the MCP_SERVERS_CONFIG section in docs/CONFIGURATION.md first, then compare the MCP architecture pages and docs/release/known-limitations.md. Document how to enable MCP tools in the production image, including pip install -e ".[mcp]", the environment variable, and the image limitation; link the FAQ from the known-limitations page and verify there is no behavior change.

Written by the indexing model from the issue text.