[Feature] Git integration helpers (Commit Message & PR Describer)

[hahwul]

Help bridge the gap between cjules and developer's local Git workflow.

Proposed commands:

1. cjules commit-msg: Generate a meaningful commit message template by sending the current git diff to Jules.
2. cjules pr-describe <id>: Generate a comprehensive pull request description (Markdown format) based on the session's actions and output patch.

Example:

cjules pr-describe <session-id> --copy # Copies generated PR description to clipboard