Replace mock git history with real `git log` data via new `get_file_history`
Tauri command. Adds git.rs module with commands for file history, modified files,
file diff, commit, and push (all registered, later tasks wire up the UI).
Updates GitCommit type to include shortHash field.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
