fix(recipe-upgrade): default to extending an existing open upgrade PR, not a parallel one

When an open upgrade PR already exists for a recipe (branch upgrade-*), push
the new work onto ITS branch and update+re-test that PR — one evolving
upgrade PR per recipe instead of spawning a second parallel PR. Only open a
fresh upgrade-<version> PR when none exists. Unrelated open PRs (e.g. backup
fixes) are still never touched; merged-upstream PRs still close.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude/skills/recipe-upgrade/SKILL.md

@@ -76,9 +76,11 @@ ssh cc-ci "GITEA_USERNAME='$GITEA_USERNAME' GITEA_PASSWORD='$GITEA_PASSWORD' GIT
    < /srv/cc-ci/.claude/skills/recipe-upgrade/open-recipe-pr.sh
 ```
 This (a) re-syncs mirror `main` to true upstream main, (b) **closes any open PR already merged
-upstream** (merging it would be a no-op), then pushes the upgrade branch and opens the new PR
-**alongside** any other still-open PRs — it does **NOT** close superseded/unrelated open PRs; the
-operator decides which to merge/close. The new PR's diff is still exactly this upgrade against the
+upstream** (merging it would be a no-op), then handles the upgrade PR: **if an open upgrade PR already
+exists for this recipe (branch `upgrade-*`), the new work is pushed onto THAT PR's branch and the PR is
+updated + re-tested** — one evolving upgrade PR per recipe, not a second parallel one. Only if none
+exists is a fresh `upgrade-<version>` PR opened. Unrelated open PRs (e.g. a backup fix) are **NOT**
+closed or touched — the operator decides on those. The PR's diff is exactly this upgrade against the
 real current upstream main. Capture the `PR_URL`. Optionally export `RECIPE_PR_BODY` first (image-tag
 table + the planned operator-action notes). Re-running with the same target version updates the
 existing same-branch PR rather than duplicating it.
@@ -152,7 +154,8 @@ Always state explicitly that **nothing was merged** — the PR(s) await operator
   `/upgrade-all` cron always runs the **default** — it never auto-updates tests.
 - **Mirror reflects reality.** Each run force-syncs the `recipe-maintainers/<recipe>` mirror `main` to
   true upstream main and closes open PRs already merged upstream. It does NOT close superseded/unrelated
-  open PRs — a new upgrade PR is opened alongside them and the operator decides which to merge/close.
+  open PRs. **An existing open upgrade PR is extended (new work pushed onto its branch + re-tested)
+  rather than spawning a second upgrade PR** — one evolving upgrade PR per recipe.
 - **Prefer a recipe-only PR.** Only open a cc-ci test PR (under `--with-tests`) when the upgrade is
   correct but a test is genuinely stale — never to paper over a real upgrade regression.
 - **Never weaken a test**; **bounded** changes (the upgrade + minimal test update, not rewrites).