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/ext/extension-api/src/contributors.rs

@@ -1,8 +1,8 @@
 use std::future::Future;
 use std::sync::Arc;
 
+use codex_context_fragments::ContextualUserFragment;
 use codex_protocol::items::TurnItem;
-use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::ReviewDecision;
 use codex_protocol::protocol::TokenUsageInfo;
 use codex_tools::ToolCall;
@@ -98,14 +98,14 @@ pub trait TurnLifecycleContributor: Send + Sync {
 /// host, not in this input.
 #[async_trait::async_trait]
 pub trait TurnInputContributor: Send + Sync {
-    /// Returns additional model input items for one submitted turn.
+    /// Returns additional contextual fragments for one submitted turn.
     async fn contribute(
         &self,
         input: TurnInputContext,
         session_store: &ExtensionData,
         thread_store: &ExtensionData,
         turn_store: &ExtensionData,
-    ) -> Vec<ResponseItem>;
+    ) -> Vec<Box<dyn ContextualUserFragment + Send>>;
 }
 
 /// Contributor for host-owned configuration changes.