rename spawn_csv feature flag to enable_fanout (#14475)

## Summary
- rename the public feature flag for `spawn_agents_on_csv()` from
`spawn_csv` to `enable_fanout`
- regenerate the config schema so only `enable_fanout` is advertised
- keep the behavior the same: enabling `enable_fanout` still pulls in
`multi_agent`

## Notes
- this is a hard rename with no `spawn_csv` compatibility alias
- the internal enum remains `Feature::SpawnCsv` to keep the patch small

## Testing
- `cd codex-rs && just fmt`
- `cd codex-rs && cargo test -p codex-core` (running locally;
`suite::agent_jobs::*` and rename-specific coverage passed so far)

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

@@ -5350,7 +5350,7 @@ async fn feature_requirements_normalize_runtime_feature_mutations() -> std::io::
 }
 
 #[tokio::test]
-async fn feature_requirements_reject_legacy_aliases() {
+async fn feature_requirements_reject_collab_legacy_alias() {
     let codex_home = TempDir::new().expect("tempdir");
 
     let err = ConfigBuilder::default()