fix: address review — restore +x, guard double comment, update stale docs (#352)

- Restore executable bit on gardener/gardener-poll.sh (cron invokes it directly)
- Add _BLOCKED_POSTED guard to prevent duplicate diagnostic comments when
  both _on_phase_change(PHASE:crashed) and the belt-and-suspenders exit
  handler both call post_blocked_diagnostic()
- Update stale documentation:
  - gardener-run.sh: remove "CI escalation recipes" from issue body
  - AGENTS.md: update directory layout comment for gardener-poll.sh
  - gardener-poll.sh: remove recipe engine description from header

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

dev/phase-handler.sh

@@ -93,6 +93,7 @@ ${tmux_output}
       -d "{\"labels\":[${blocked_id}]}" >/dev/null 2>&1 || true
   fi
   CLAIMED=false
+  _BLOCKED_POSTED=true
 }
 
 # --- Build phase protocol prompt (shared across agents) ---