feat(session-inbox-monitor): close -test-trigger (PASS) + add CLAUDE.md trigger-line

test-trigger run in a clean session via 7 unprimed clean-context subagents.
Positives 4/4 -> session-inbox-monitor (incl. CLAUDE.md-line P4).
Negatives 2/3 clean (N2 multi-machine backend, N3 received-msg EN -> none);
N1 RU received-msg = borderline false-positive, self-corrects on body-load,
root cause = inter-session-peer-discipline sibling not installed in registry.
Filed follow-up session-inbox-monitor-received-msg-fp. -review unblocked
(needs a non-implementer session). Added `inbox monitor: raise on start` to
project CLAUDE.md (user-approved) -- discoverable opt-in + materializes P4.

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

CLAUDE.md

@@ -8,6 +8,7 @@ use task management system
 check across all projects
 pull remote before work
 session handoff: read on start, write on end
+inbox monitor: raise on start
 follow project discipline
 follow tdd-criteria
 delegate to interns when allowed