feat: gate reconciliation audit mode from unauthorized cleanup (Closes #419)

Add audit vs cleanup phase tracking so reconciliation audits stay read-only
unless cleanup is explicitly authorized with delete capability proof, safety
proof, and before/after snapshots. Block gitea_delete_branch and merged-cleanup
execution during audit phase, validate final reports for false no-mutations
claims, and document the boundary in the reconcile-landed-pr workflow.

task_capability_map.py

@@ -104,6 +104,10 @@ TASK_CAPABILITY_MAP: dict[str, dict[str, str]] = {
         "permission": "gitea.read",
         "role": "author",
     },
+    "reconciliation_cleanup": {
+        "permission": "gitea.branch.delete",
+        "role": "author",
+    },
     "work_issue": {
         "permission": "gitea.pr.create",
         "role": "author",