feat: push notes to remote GitHub repo on save

[kwannoel]

Summary

• Add push_to_remote() to notes module — checks for origin remote via git2, shells out to git push origin HEAD if present
• Integrate into command layer so every note commit triggers a background push
• Show error toast on sync failure via notes-sync-error Tauri event

Setup

cd ~/.the-controller/notes
git remote add origin git@github.com:user/my-notes.git

Test plan

• push_to_remote returns Ok when no remote configured
• push_to_remote pushes to bare repo remote in test
• All 7 note mutation commands trigger background push
• commit_notes triggers push when content changes
• Frontend shows toast on sync error
• All Rust tests pass (399+)
• All frontend tests pass (291)

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