feat(grpc): populate commit SHA when loading catalog via gRPC

[juliuskrah]

Background

GRPCLoader.LoadFromTag initialises the catalog with an empty commit SHA:

// gitstore-api/internal/catalog/grpc_loader.go:41
cat := NewCatalog("", tag)

ListFilesResponse from the git-service already includes ref_commit_sha, but the loader discards it. As a result catalog.Commit() always returns "", which:

• Produces empty commit fields in cache-reload log lines (zap.String("commit", ...) in cache/manager.go)
• Breaks any future client logic that expects a concrete commit hash for traceability or cache keying

Proposed fix

Capture ref_commit_sha from the first ListFiles response (all three prefix calls resolve to the same ref, so any one is sufficient) and pass it to NewCatalog:

cat := NewCatalog(refCommitSHA, tag)

Alternatively, call GetLatestTag (which already returns commit_sha on the TagEntry) and use that value directly in LoadFromLatestTag before delegating to LoadFromTag.

Acceptance criteria

• catalog.Commit() returns a non-empty 40-char SHA after a gRPC-backed reload
• Existing unit and integration tests continue to pass
• Cache reload log line includes the resolved commit SHA

Identified during review of #122.