- Add GET /api/matrix/thoughts endpoint that returns recent thoughts
- Returns last 10 thoughts by default, with ?limit=N param (max 50)
- Each thought has: id, text (truncated to 500 chars), created_at, chain_id
- Reuses existing thinking_engine.get_recent_thoughts() logic
- Returns empty array if thinking engine is disabled or fails
- Add comprehensive test coverage
Fixes#677