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/context/plugin_instructions.rs

@@ -12,7 +12,7 @@ impl PluginInstructions {
 }
 
 impl ContextualUserFragment for PluginInstructions {
-    fn role() -> &'static str {
+    fn role(&self) -> &'static str {
         "developer"
     }