fix: DELETE /issues/{id}/labels/backlog uses label name not numeric ID (#214)

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

dev/phase-handler.sh

@@ -570,7 +570,7 @@ A maintainer should split this issue or add more detail to the spec."
             -d "{\"labels\":[${UNDERSPECIFIED_LABEL_ID}]}" >/dev/null 2>&1 || true
           curl -sf -X DELETE \
             -H "Authorization: token ${CODEBERG_TOKEN}" \
-            "${API}/issues/${ISSUE}/labels/backlog" >/dev/null 2>&1 || true
+            "${API}/issues/${ISSUE}/labels/${BACKLOG_LABEL_ID}" >/dev/null 2>&1 || true
           notify "refused #${ISSUE}: too large — ${REASON}"
           ;;
         already_done)