Wrap git operations in GitClient interface

Create internal/daemonrunner/git.go with GitClient interface (HasUpstream, HasChanges, Commit, Push, Pull). Default implementation using os/exec. Use in Syncer and Run loop for testability.


Document bd edit command and verify MCP exclusion

Follow-up from PR #152:
1. Add "bd edit" to AGENTS.md with "Humans only" note
2. Verify MCP server doesn't expose bd edit command
3. Consider adding test for command registration