fix: phase-handler.sh DELETE backlog label uses string 'backlog' not numeric ID (#375)

Replace string 'backlog' with $BACKLOG_LABEL_ID in the DELETE label API call
at dev-agent.sh:370. The Gitea API expects a numeric label ID in that path
segment. The variable is already resolved at the top of the script (line 46-48)
and used correctly elsewhere.

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

dev/dev-agent.sh

@@ -367,7 +367,7 @@ curl -sf -X POST \
 
 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
 
 CLAIMED=true