fix: address round 2 review findings from issue #81

- Move atomic mv inside gardener loop so reply is only claimed when a
  matching needs_human session exists (fixes reply-loss regression)
- Delay rm of claimed file until after successful injection in both
  supervisor and gardener (OOM/SIGKILL leaves file recoverable)
- Fix matrix_listener ack message: 'next poll' instead of 'next supervisor poll'

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

lib/matrix_listener.sh

@@ -143,7 +143,7 @@ while true; do
         ;;
       dev)
         printf '%s\t%s\t%s\n' "$(date -u +%Y-%m-%dT%H:%M:%SZ)" "$SENDER" "$BODY" >> /tmp/dev-escalation-reply
-        matrix_send "dev" "✓ received, will inject on next supervisor poll" "$THREAD_ROOT" >/dev/null 2>&1 || true
+        matrix_send "dev" "✓ received, will inject on next poll" "$THREAD_ROOT" >/dev/null 2>&1 || true
         ;;
       vault)
         # Parse APPROVE <id> or REJECT <id> from reply