Run the org PR labeler on .github's own PRs

Add a thin caller so the .github repo invokes its own reusable
callable-labeler.yaml on pull_request, like every consumer repo does. Without a
caller the workflow_call-only labeler never runs on .github's own PRs (this is
why #61 wasn't auto-labeled). Grants the three permissions the reusable requires
(contents:read, pull-requests:write, issues:write).

.github/workflows/labeler.yaml

@@ -0,0 +1,25 @@
+name: labeler
+
+# Thin caller that runs the org-wide reusable PR labeler (callable-labeler.yaml)
+# on THIS repo's own pull requests. The .github repo is the single source of truth
+# for the reusable workflows, but — like any consumer repo — it must invoke them
+# via a caller to use them on itself; without this, the labeler never runs on
+# .github's own PRs (the reusable is `workflow_call`-only).
+#
+# Permissions are load-bearing: callers MUST grant all three below. Reusable-
+# workflow permissions can only be downgraded from the caller, so omitting one
+# (e.g. issues:write) either fails to create labels or triggers a silent
+# startup_failure. `pull_request` (NOT pull_request_target) is correct here — the
+# org takes no fork PRs, so the lower-privilege event is sufficient.
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+  pull-requests: write
+  issues: write
+
+jobs:
+  label:
+    uses: ./.github/workflows/callable-labeler.yaml