fix: extract restore_to_backlog() to eliminate duplicate label reset pattern

The cleanup_labels + curl POST + CLAIMED=false pattern was duplicated
across dev-agent.sh (idle_timeout and crashed cases) and phase-handler.sh
(PHASE:crashed handler), triggering duplicate-detection CI failure.

Extract restore_to_backlog() shared helper; call it from all three sites.

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

dev/phase-handler.sh

@@ -726,14 +726,7 @@ $(printf '%s' "$REFUSAL_JSON" | head -c 2000)
       >> "${FACTORY_ROOT}/supervisor/escalations-${PROJECT_NAME}.jsonl"
 
     # Restore backlog label so issue can be retried
-    cleanup_labels
-    curl -sf -X POST \
-      -H "Authorization: token ${CODEBERG_TOKEN}" \
-      -H "Content-Type: application/json" \
-      "${API}/issues/${ISSUE}/labels" \
-      -d "{\"labels\":[${BACKLOG_LABEL_ID}]}" >/dev/null 2>&1 || true
-
-    CLAIMED=false  # Don't unclaim again in cleanup()
+    restore_to_backlog
     if [ -n "${PR_NUMBER:-}" ]; then
       log "keeping worktree (PR #${PR_NUMBER} still open)"
     else