Disable automatic caching for sensitive events (#992)

## Summary

- disable `enable-cache: auto` for `pull_request_target`,
`workflow_run`, and `release` events
- disable automatic caching for tag pushes while leaving branch pushes
unchanged
- preserve explicit `enable-cache: true` as an override
- run a `workflow_run` integration fixture with `act` in pull request CI
and verify caching is disabled
- document the behavior and update the published bundles

## Testing

- `npm run all`
- `actionlint .github/workflows/test.yml
__tests__/workflows/workflow-run.yml`
- `uvx zizmor __tests__/workflows/workflow-run.yml`

Closes #984

Refs: pi-session 019fec42-9b26-714e-a359-830ac4401ecd

README.md

@@ -74,7 +74,7 @@ Have a look under [Advanced Configuration](#advanced-configuration) for detailed
     # Used when downloading uv from GitHub releases
     github-token: ${{ github.token }}
 
-    # Enable uploading of the uv cache: true, false, or auto (enabled on GitHub-hosted runners, disabled on self-hosted runners)
+    # Enable the GitHub Actions cache for uv: true, false, or auto (enabled on GitHub-hosted runners except for release, tag push, pull_request_target, and workflow_run events; disabled on self-hosted runners)
     enable-cache: "auto"
 
     # Glob pattern to match files relative to the repository root to control the cache