Problem

Gitlink tree entries (mode 160000) are parsed but there is no submodule resolution, .gitmodules parsing, or recursive handling. Submodule entries are opaque SHA references with no way to follow them.

Impact

• Repositories with submodules can be stored but not fully traversed
• Diff/merge cannot meaningfully handle submodule changes
• Tree walks silently skip submodule content

Acceptance Criteria

• .gitmodules file parsing (URL, path, branch mappings)
• API to resolve submodule SHAs to their source repository
• Tree walks can optionally recurse into submodules
• Diff reports submodule SHA changes with context (old → new commit)
• Tests with realistic submodule structures