fix(importer): clean untracked files after checkout in reconciler

[another-rex]

In handleReconcileGit, after checking out the target branch, clean up any untracked files and directories to ensure a clean state and avoid processing stale/deleted files.

This extends commit 828810a.

Key Changes:

• System Git Refactor: Refactored handleReconcileGit to use system git checkout and git clean -fdx commands instead of go-git library calls. This is much more robust and avoids potential out-of-memory or timeout issues with go-git's Status() on large repositories when deployed.
• Concurrency Fix: Moved checkout and clean out of repoGroup.Do and protected them with repo.mu lock to avoid incorrect state when concurrent imports and reconciles occur.
• Testing: Added TestHandleReconcileGit_CleanUntracked to verify that untracked files and directories are correctly cleaned up.

agy

[another-rex]

I'm not sure whether we should even have this test.