feat(mcp): auto workspace switching, ambient team updates, batch locks, optional agentId

Three friction fixes from live multi-agent field reports:

- Dynamic workspace switching: every tool call re-resolves the workspace
  from runtime hints and absolute path arguments; disjoint-root detection
  rebinds the session in-process (no more stale "default" board until
  restart). Monorepo subpackages never trigger a false switch.
- Ambient team context: per-agent notepad cursors deliver what OTHER
  agents logged since your last coordination call as a capped trailer on
  12 coordination tools — no polling, no new tool to remember.
- Less ceremony: agentId is optional everywhere (defaults to the session
  identity it was being normalized onto anyway), and propose_file_access
  accepts filePaths[] for all-or-nothing batch locking in one round-trip.

26 new tests; suite 114 pass / 0 fail; tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>