feat: add deterministic advisory Board watcher

[bearmug]

Summary

Closes #409.

• Add a bounded deterministic Board watcher with off, shadow, and explicit intervene modes.
• Wire the watcher into existing session lifecycle evidence without invoking models.
• Queue non-binding advisor:board messages as nextTurn only; no automatic turn, model switch, tool call, or mutation occurs.
• Add deduplication, cooldown, intervention caps, session state, status controls, and completions.

Validation

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

Stack

Base of the advisory posture stack. #410 will add bounded Head-of-Board escalation; #411 will document cheap main-model profiles and runtime model truth.