feat: escalate Board risks to read-only Head

[bearmug]

Summary

Closes #410.

• Add opt-in bounded Board-to-Head escalation on material risks.
• Reuse the compact sanitized Board ledger and the existing read-only Head adapter.
• Cap Head calls per session, use one completion attempt, deduplicate risk fingerprints, and fail closed on errors.
• Queue Head output as visible non-binding nextTurn advice with no automatic turn or mutation.
• Add explicit board watch head status|on|off controls and integration coverage.

Validation

• npm test -- --run packages/advisor/src/board-watcher.test.ts packages/advisor/src/extension.test.ts — 19 tests passed
• npm run check — passed
• git diff --check — passed

Stack

Base: PR #412 / issue #409. #411 will document cheap main-model profiles and runtime model truth on top of this PR.