chore: remove Codex GitHub workflow and add commit message helper

- Deleted `.github/workflows/codex.yml` to remove the Codex GitHub workflow.
- Added a `commitmsg` task in `Justfile` to generate commit messages using
  `gh commitmsg`.
- Streamlines the repository by removing unused workflows and adding a
  utility for consistent commit message generation.

Author: hazadus

.github/workflows/codex.yml

@@ -9,3 +9,6 @@ format:
 
 lint:
   golangci-lint run ./...
+
+commitmsg:
+  gh commitmsg --examples