[spelech]

Closes #101

Summary

• Adds configurable retain_commits_count (default 20, range 0..500) to git_repositories and creates the git_commits table with (repo, commit_sha) uniqueness and indexes.
• Updates shallow_clone_repo and sync_single_git_repo to fetch with depth max(1, retain_commits_count), extract recent commit metadata via extract_recent_commits, and automatically prune older rows beyond retain_commits_count (or purge all when retain_commits_count == 0).
• Exposes get_recent_changes MCP tool returning compact JSON (including structured fallback notice when retain_commits_count == 0 or no commits are retained).
• Adds UI controls for retain_commits_count in AddRepoModal and RepoListTable.