Document Codex git commit attribution config (#21379)

## Summary
- document that commit attribution for generated git commit messages is
gated by the `codex_git_commit` feature flag
- add an example `config.toml` snippet showing `commit_attribution` with
`[features].codex_git_commit = true`
- update the config schema description so the reference docs explain
that `commit_attribution` only takes effect when the feature is enabled

Fixes #19799.

## Validation
- `cargo run -p codex-core --bin codex-write-config-schema`
- `cargo test -p codex-config`
- `cargo test -p codex-features`
- `cargo fmt --check`
- `git diff --check`

## Notes
- `cargo test -p codex-core config_schema_matches_fixture` currently
fails before reaching the schema test because `core_test_support`
imports `similar` without a linked crate in this checkout. The narrower
package checks above avoid that unrelated test-support build failure.

docs/config.md

@@ -5,3 +5,24 @@ For basic configuration instructions, see [this documentation](https://developer
 For advanced configuration instructions, see [this documentation](https://developers.openai.com/codex/config-advanced).
 
 For a full configuration reference, see [this documentation](https://developers.openai.com/codex/config-reference).
+
+## Commit attribution
+
+Codex can add a [git trailer](https://git-scm.com/docs/git-interpret-trailers) to
+generated commit messages so commits make Codex's involvement explicit. This
+behavior is gated by the `codex_git_commit` feature flag; the top-level
+`commit_attribution` setting is only used when that feature is enabled.
+
+Add the following to `~/.codex/config.toml`:
+
+```toml
+commit_attribution = "Codex <noreply@openai.com>"
+
+[features]
+codex_git_commit = true
+```
+
+When enabled, Codex appends a `Co-authored-by:` trailer using the configured
+attribution value. If `commit_attribution` is omitted, Codex uses
+`Codex <noreply@openai.com>`. Set `commit_attribution = ""` to disable the
+trailer while leaving the feature flag enabled.