feat(actions): add branch filters to run list (#37826)

## Summary

- Add a Branch filter dropdown to the repo Actions run list web UI
- Wire `?branch=` query param through the web handler, matching the
existing REST API filter behavior
- Source the Branch dropdown from the indexed `branch` table (filtering
out deleted branches) instead of scanning `action_run.ref`, addressing
review feedback about unindexed columns

The Event filter was dropped after review: a static list of supported
events was noisy as UX, and querying distinct values from
`action_run.trigger_event` is slow because the column is not indexed.
`FindRunOptions.TriggerEvent` is kept for the REST API.

Closes #25042

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Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>