feat: Add auto-snapshot feature with git commit every N words

- Add new module src/app/auto_snapshot.rs for auto-snapshot functionality
- Add configuration options: auto_snapshot_enabled (default: false) and
  auto_snapshot_threshold (default: 250 words)
- Automatically create git commits when word count crosses threshold boundaries
- Add 'Edit -> Snapshots' menu option to open snapshots window
- Snapshots window shows: enable/disable toggle, threshold setting, current
  word count, next snapshot target, list of all snapshots with rollback buttons
- Manual snapshot creation via button in snapshots window
- Rollback to any previous snapshot with one click
- Snapshot info includes: word count, timestamp, commit message, commit hash
- Make run_git in gitsync.rs public for use by auto_snapshot module
- Update README.md with Auto-Snapshots documentation

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/gitsync.rs

@@ -7,7 +7,7 @@ pub struct GitOutcome {
     pub log: String,
 }
 
-fn run_git(workspace: &Path, args: &[&str]) -> (bool, String) {
+pub fn run_git(workspace: &Path, args: &[&str]) -> (bool, String) {
     let output = Command::new("git")
         .current_dir(workspace)
         .args(args)