fix: feat: gardener recycles stale failed PRs back to backlog (#626)

Add stale-pr-recycle step to the gardener formula that detects open PRs
with failed CI older than 24 hours and no active tmux session. Stale PRs
are closed with a comment, and the linked issue is relabeled from
in-progress to backlog so dev-poll picks it up for a fresh attempt.

Also adds close_pr manifest action to the gardener executor.

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

gardener/gardener-run.sh

@@ -86,6 +86,7 @@ Supported actions:
   {\"action\":\"comment\",      \"issue\":NNN, \"body\":\"Relates to issue 1031\"}
   {\"action\":\"create_issue\", \"title\":\"...\", \"body\":\"...\", \"labels\":[\"backlog\"]}
   {\"action\":\"edit_body\",    \"issue\":NNN, \"body\":\"new body\"}
+  {\"action\":\"close_pr\",    \"pr\":NNN}
 
 The commit-and-pr step converts JSONL to JSON array. The orchestrator executes
 actions after the PR merges. Do NOT call mutation APIs directly during the run."
@@ -274,6 +275,19 @@ _gardener_execute_manifest() {
         fi
         ;;
 
+      close_pr)
+        local pr
+        pr=$(jq -r ".[$i].pr" "$manifest_file")
+        if curl -sf -X PATCH -H "Authorization: token ${FORGE_TOKEN}" \
+             -H 'Content-Type: application/json' \
+             "${FORGE_API}/pulls/${pr}" \
+             -d '{"state":"closed"}' >/dev/null 2>&1; then
+          log "manifest: closed PR #${pr}"
+        else
+          log "manifest: FAILED close_pr #${pr}"
+        fi
+        ;;
+
       *)
         log "manifest: unknown action '${action}' — skipping"
         ;;