[E7-3] Monitor loop bounded concurrency — serial checkSession for 200 sessions

**Finding:** SC-3 (MEDIUM) | **Milestone:** M-E7: Scalability (#21)

**Problem:** checkSession runs serially for all 200 sessions per poll cycle.

**Fix:** Run checkSession with bounded concurrency (p-limit 20) instead of Promise.all or serial.

**Files:** src/monitor.ts

**Acceptance:** Monitor poll cycle completes in under 2s for 100 active sessions.