feat(api): extend commit and diff services with new functionality

- Add FindCommit, ListCommitsByOid, CommitIsAncestor RPCs to CommitService
- Add CheckObjectsExist, CommitsByMessage, GetCommitStats RPCs to CommitService
- Add LastCommitForPath, CountCommits, CountDivergingCommits RPCs to CommitService
- Add RawDiff, RawPatch, FindChangedPaths RPCs to DiffService
- Add FindMergeBase, WriteRef, SearchFilesByContent RPCs to RepositoryService
- Add SearchFilesByName, ObjectsSize, RepositorySize RPCs to RepositoryService
- Add FindLicense, OptimizeRepository, GetRawChanges RPCs to RepositoryService
- Add FetchRemote, CreateRepositoryFromURL RPCs to RepositoryService
- Implement server handlers for all new RPC methods
- Add new modules for commit counting, finding, and querying features
- Add new modules for diff changed paths and raw operations
- Add new modules for refs and remote operations
- Remove unnecessary comments from various source files
- Update proto definitions with new message types and service methods

main.rs

@@ -125,12 +125,10 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
         "health check: interval={health_check_interval}s, max_failures={max_health_failures}"
     );
 
-    // ── Metrics server ──
     let metrics_port = env_u64("GITKS_METRICS_PORT", 9100) as u16;
     let _metrics_handle = metrics::start_metrics_server(metrics_port);
     tracing::info!("metrics server on port {metrics_port}");
 
-    // ── Cluster discovery (etcd → ractor_cluster) ──
     //
     // When GITKS_ETCD_ENDPOINTS is set, the node:
     //   1. Starts a ractor_cluster NodeServer (TCP listener)