skills: resolve per-turn catalogs from turn input context (#26106)

## Why

The skills extension needs the resolved turn environments to build a
real per-turn `SkillListQuery`. The previous `TurnLifecycleContributor`
hook only had a turn id, so it could only seed a placeholder query and
never carry the executor authorities that executor-scoped skill routing
will need.

Moving catalog resolution onto `TurnInputContributor` puts the skills
extension on the same turn-preparation path that already has the
environment ids and working directories for the submitted turn, while
keeping the actual prompt injection work for follow-up changes.

## What changed

- switch `ext/skills` from `TurnLifecycleContributor` to
`TurnInputContributor`
- build `executor_authorities` from `TurnInputContext.environments` and
pass them through `SkillListQuery`
- keep storing the resolved catalog in `SkillsTurnState`, but drop the
placeholder query helper that no longer matches the real data flow
- update the extension TODOs to reflect that per-turn catalog resolution
now happens in the turn-input contributor, and that prompt/context
injection still needs to move later

## Testing

- Not run locally.

codex-rs/core/src/session/turn.rs

@@ -628,7 +628,7 @@ async fn build_extension_turn_input_items(
 
     let mut items = Vec::new();
     for contributor in contributors {
-        let contributed_items = contributor
+        let contributed_fragments = contributor
             .contribute(
                 input.clone(),
                 &sess.services.session_extension_data,
@@ -638,7 +638,11 @@ async fn build_extension_turn_input_items(
             .or_cancel(cancellation_token)
             .await
             .ok()?;
-        items.extend(contributed_items);
+        items.extend(
+            contributed_fragments
+                .into_iter()
+                .map(ContextualUserFragment::into_boxed_response_item),
+        );
     }
 
     Some(items)