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/features.rs

@@ -709,7 +709,7 @@ pub const FEATURES: &[FeatureSpec] = &[
     },
     FeatureSpec {
         id: Feature::SpawnCsv,
-        key: "spawn_csv",
+        key: "enable_fanout",
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },