ci(cla): report dispatched CLA pass via commit status

[teamchong]

#204 wasn't enough. When cla.yml runs via workflow_dispatch, the run's check suite lands on the commit with associated_prs=0, so it never attaches to the release PR. The run goes green but the merge box never sees a CLAssistant check and the PR stays blocked on the required status check.

This keeps everything from #204 (trigger, validation guard, dispatch from release.yml) and only changes how the result is reported. After the guard passes, the workflow posts a commit status with context CLAssistant on $GITHUB_SHA. Statuses attach to the SHA directly, so they show up in the PR status rollup and satisfy the required check even with the ruleset pinned to the GitHub Actions app. No new permissions, the workflow already had statuses: write.

Tested end to end on my fork instead of experimenting here. Mirrored the ruleset (required status check CLAssistant pinned to the GitHub Actions app) and had a throwaway workflow open a fake release PR shaped like a real one, opened by github-actions[bot] from changeset-release/main with a bot-authored head commit so it passes the guard: teamchong#2. Before the fix it sat at mergeStateStatus: BLOCKED with an empty rollup, same as the incident here. Then I dispatched it the same way release.yml does (gh workflow run cla.yml --ref changeset-release/main -f pull_request=2, run: https://github.com/teamchong/capnweb/actions/runs/28710748911) and it flipped to mergeable: true with CLAssistant: success in the rollup.

[changeset-bot]

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Latest commit: 200c4b0

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commit: 200c4b0