Four optional, serde-skipped-when-None fields added to `Metadata` for code ingest context. All 11 downstream construction sites patched with `repo: None, git_branch: None, git_commit: None, code_lang: None`. Full workspace check (`--tests`) and per-crate test suite pass clean. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>