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.

codex-rs/core/src/config/mod.rs

@@ -476,6 +476,8 @@ pub struct Config {
     pub compact_prompt: Option<String>,
 
     /// Optional commit attribution text for commit message co-author trailers.
+    /// This top-level setting only takes effect when `[features].codex_git_commit`
+    /// is enabled.
     ///
     /// - `None`: use default attribution (`Codex <noreply@openai.com>`)
     /// - `Some("")` or whitespace-only: disable commit attribution