Update CHANGELOG for v9.4.0 release (#851)

### Summary

Create new v9.4.0 section in CHANGELOG with the November 22, 2025
release. Move the PR #850 entry (SHAKAPACKER_SKIP_PRECOMPILE_HOOK
environment variable) from Unreleased to v9.4.0. Fix the placeholder PR
number and update version diff links.

### Pull Request checklist

- [x] Update CHANGELOG file
- ~[ ] Add/update test to cover these changes~
- ~[ ] Update documentation~

### Other Information

This changelog update reflects the v9.4.0 release that was tagged on
November 22, 2025. The Unreleased section is now empty and ready for
future changes.

Co-authored-by: Claude <[email protected]>

Author: justin808

CHANGELOG.md

@@ -11,6 +11,8 @@
 
 Changes since the last non-beta release.
 
+## [v9.4.0] - November 22, 2025
+
 ### Added
 
 - **Added `SHAKAPACKER_SKIP_PRECOMPILE_HOOK` environment variable to skip precompile hook**. [PR #850](https://github.com/shakacode/shakapacker/pull/850) by [justin808](https://github.com/justin808). Set `SHAKAPACKER_SKIP_PRECOMPILE_HOOK=true` to skip the precompile hook during compilation. This is useful when using process managers like Foreman or Overmind to run the hook once before starting multiple webpack processes, preventing duplicate hook execution. **Migration tip:** If you have a custom `bin/dev` script that starts multiple webpack processes, you can now run the precompile hook once in the script and set this environment variable to prevent each webpack process from running the hook again. See the [precompile hook documentation](./docs/precompile_hook.md#skipping-the-hook) for implementation examples.
@@ -763,7 +765,8 @@ Note: [Rubygem is 6.3.0.pre.rc.1](https://rubygems.org/gems/shakapacker/versions
 
 See [CHANGELOG.md in rails/webpacker (up to v5.4.3)](https://github.com/rails/webpacker/blob/master/CHANGELOG.md)
 
-[Unreleased]: https://github.com/shakacode/shakapacker/compare/v9.3.4-beta.0...main
+[Unreleased]: https://github.com/shakacode/shakapacker/compare/v9.4.0...main
+[v9.4.0]: https://github.com/shakacode/shakapacker/compare/v9.3.4...v9.4.0
 [v9.3.4-beta.0]: https://github.com/shakacode/shakapacker/compare/v9.3.3...v9.3.4-beta.0
 [v9.3.3]: https://github.com/shakacode/shakapacker/compare/v9.3.2...v9.3.3
 [v9.3.2]: https://github.com/shakacode/shakapacker/compare/v9.3.1...v9.3.2