fix: Session lock must not block during idle phases (awaiting_review/awaiting_ci) (#724)

Restructure session.lock from command-wrapper flock to fd-based flock so
the lock can be released when Claude is idle and re-acquired before
injecting the next prompt.

- agent-session.sh: add session_lock_acquire/release helpers, open fd in
  create_agent_session instead of wrapping claude with flock, auto-acquire
  in agent_inject_into_session before injecting
- phase-handler.sh: call session_lock_release at start of awaiting_ci and
  awaiting_review handlers (Claude is idle during CI polling / review wait)

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

dev/phase-handler.sh

@@ -261,6 +261,8 @@ _on_phase_change() {
 
   # ── PHASE: awaiting_ci ──────────────────────────────────────────────────────
   if [ "$phase" = "PHASE:awaiting_ci" ]; then
+    # Release session lock — Claude is idle during CI polling (#724)
+    session_lock_release
 
     # Create PR if not yet created
     if [ -z "${PR_NUMBER:-}" ]; then
@@ -458,6 +460,8 @@ Instructions:
 
   # ── PHASE: awaiting_review ──────────────────────────────────────────────────
   elif [ "$phase" = "PHASE:awaiting_review" ]; then
+    # Release session lock — Claude is idle during review wait (#724)
+    session_lock_release
     status "waiting for review on PR #${PR_NUMBER:-?}"
     CI_FIX_COUNT=0  # Reset CI fix budget for this review cycle