Cancel stale On PR runs so only current HEAD determines status (#775)

Adds a concurrency group on the On PR workflow with
cancel-in-progress: true.

Context: the upstream-release-docs workflow pushes multiple
commits in quick succession to a Renovate PR branch — the skill's
content commit, then a prettier/eslint autofix commit that cleans
any formatting drift the skill introduced. Without concurrency,
the On PR workflow runs fully on each intermediate commit and the
first one (pre-autofix) can fail lint. The failure lingers in the
check history even though the final commit passes.

With cancel-in-progress, when the autofix commit lands, the stale
On PR run on the skill commit is cancelled. Only the final run
determines the check status. Matches standard GitHub Actions
practice for PR CI.

Concurrency group is keyed on pull_request.number so PRs can't
cancel each other's runs.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: rdimitrov

.github/workflows/on-pr.yaml

@@ -6,6 +6,17 @@ on:
 permissions:
   contents: read
 
+# Cancel stale runs so only the PR's current HEAD determines the
+# check status. Matters specifically for PRs where the
+# upstream-release-docs workflow pushes multiple commits
+# in quick succession (skill output + prettier/eslint autofix): the
+# intermediate commits briefly fail lint until the autofix lands,
+# and without cancel-in-progress those intermediate failures stay
+# in the check history.
+concurrency:
+  group: on-pr-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
 jobs:
   static-checks:
     name: Static checks