fix: fix: phase-handler.sh references LAST_PHASE_MTIME which is now internal to monitor_phase_loop (#181)

Export LAST_PHASE_MTIME from monitor_phase_loop before invoking the callback
so that phase-handler.sh can compare phase file mtimes inside the awaiting_review
inner poll loop without hitting an unbound variable error under set -u.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/agent-session.sh

@@ -114,6 +114,8 @@ monitor_phase_loop() {
 
     # Phase changed
     last_mtime="$phase_mtime"
+    # shellcheck disable=SC2034  # read by phase-handler.sh callback
+    LAST_PHASE_MTIME="$phase_mtime"
     idle_elapsed=0
 
     # Terminal phases