Add support for `push` events as an automation event type, enabling
workflows triggered by pushes to use claude-code-action.
Changes:
- Add PushEvent to automation event types in context.ts
- Add case handler for push events in parseGitHubContext()
- Add isPushEvent() type guard
- Add test for push event mode detection
This enables use cases like auto-rebasing PRs when main is updated,
where a workflow triggers on push to main and uses a prompt to
instruct Claude to merge main into stale PR branches.
8151408b90