Define typed runner-controlled gitlink authoring #7

V1 reviews externally produced gitlink metadata but cannot author submodule-pointer changes under its schema and read-only agent .git policy. Before enabling authoring, define a typed target commit and validated runner-controlled Git entry operation in a new schema version. Preserve empty/read-only nested worktrees and verify target object identity. Add schema migration, import, and post-run enforcement tests. See PR #1 discussion 4078808289.