feat(release): rebase PR branch before updating changelog

When updating an existing release PR, the workflow now rebases
the next-release branch onto the base branch using Gitea's PR
update API with style=rebase. This keeps the PR up to date with
main without force-pushing or deleting the branch.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/settings.local.json

@@ -2,7 +2,10 @@
   "permissions": {
     "allow": [
       "Bash(but --help:*)",
-      "Bash(but rub --help:*)"
+      "Bash(but rub --help:*)",
+      "WebSearch",
+      "WebFetch(domain:docs.gitea.com)",
+      "WebFetch(domain:gitea.com)"
     ]
   }
 }