fix(monitor): clean per-session Maps on signal shutdown (#1115)

[OneStepAt4time]

Bug: signal-cleanup-helper.ts called sessions.killSession but did NOT call cleanupTerminatedSessionState. When SIGTERM/SIGINT fired, all monitor/metrics/toolRegistry per-session Maps accumulated stale entries — memory leak.

Fix:

1. Added optional SessionCleanupDeps parameter to killAllSessions and killAllSessionsWithTimeout in signal-cleanup-helper.ts
2. After each killSession, call cleanupTerminatedSessionState if deps provided
3. Updated server.ts shutdown path to pass { monitor, metrics, toolRegistry }

Note: removeSession (monitor cleanup) was already comprehensive — the issue was that it was never called in the signal shutdown path.

Developed with Aegis v0.1.0-alpha

Refs: #1115