feat(p10-1a-1): add Metadata.repo / git_branch / git_commit / code_lang

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>

crates/kebab-normalize/src/lib.rs

@@ -467,6 +467,10 @@ mod tests {
             trust_level: TrustLevel::Primary,
             user_id_alias: None,
             user,
+            repo: None,
+            git_branch: None,
+            git_commit: None,
+            code_lang: None,
         }
     }