Rep stats performance and progress

[kyle-rader]

Optimize repo stats performance for large repositories and add a delayed progress meter.

The repo stats command previously loaded the entire git log into memory and processed it in-app, leading to slow performance on large codebases. This PR streams git log output, pushes date filtering directly to git using --since/--until flags, and removes an unused internal timeline data structure. A 1-second delayed, self-clearing progress spinner is also added to improve UX for long-running computations.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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