feat(commit): generate commit messages from a configured command

[chirino]

PR Description

Summary

Add a configurable commit message generator that lets users run their own script or tool from Lazygit’s commit menu. This keeps the generated message editable before commit, while supporting slower AI-backed generators with an in-progress state and cancellation.

Changes

• Add git.commit.messageGeneratorCommand, which receives the repo root as its final argument and uses stdout as the editable commit message.
• Add a Generate Commit Message commit-menu action when the command is configured.
• Show generating/canceling UI state with a spinner, disable editing while generation is running, and allow cancellation.
• Surface command failures with stderr while preserving the current commit message.
• Add integration and context tests for success, failure, cancellation, and subtitle state.
• Document the config option and provide example generator scripts, including a Codex-backed script.

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[chirino]

this is what it ends up looking like.