Git panel operations have no progress indicator

[JDonaghy]

Commit, push, and pull operations in the git panel give no visual feedback that an operation is in progress.

VSCode shows a spinner for a few seconds to confirm the action was triggered. VimCode should show a spinner (using the existing notification/progress system) while git operations are running.

Key files: `src/core/engine/source_control.rs` — `sc_do_commit()`, `sc_do_push()`, `sc_do_pull()`
Existing infrastructure: `notify()` / `notify_done_by_kind()` / `tick_notifications()` in `src/core/engine/panels.rs`