docs: spec the silent-until-signal monitor policy

An in-session monitor loop (sentry, auto triage-intake, auto inbox-zero) fires the model every interval and narrates a full turn even when nothing changed, so a session fills with quiet-fire noise. The spec settles the fix as a policy, not a watcher: detect first, and on an empty check collapse to one labelled heartbeat line, spending a full turn only on a real item. Keeping detection in-session dissolves the MCP-auth split (triage's sources need the session's auth, which a detached watcher can't reach), so it applies uniformly across all three loops. DRAFT, decisions resolved live. Links a [#B] build task.

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