fix: idle_pane_count not reset in phase-changed branch of monitor_phase_loop (#436)

When a phase change is detected (mtime changes), idle_elapsed was reset
but idle_pane_count was not. This meant idle counts accumulated before a
phase write carried into subsequent polls, so N consecutive idle polls
could be reached with fewer than N actual consecutive idle polls.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/agent-session.sh

@@ -399,6 +399,7 @@ monitor_phase_loop() {
     # shellcheck disable=SC2034  # read by phase-handler.sh callback
     LAST_PHASE_MTIME="$phase_mtime"
     idle_elapsed=0
+    idle_pane_count=0
 
     # Terminal phases
     case "$current_phase" in