Move host skill prompt injection into the skills extension (#37503)

## What changed

- Add `HostSkillsSnapshot::load_skill_prompts` so the skills extension owns reading and rendering selected host skills.
- Keep explicit invocation telemetry, unreadable-skill warnings, plugin prompt ordering, and suppression of host prompts superseded by provider prompts.
- Remove the legacy prompt injection and fragment implementation from `codex-core-skills`.

## Testing

- Cover prompt truncation without an installed skills extension, skill/plugin ordering, selective provider supersession, unreadable skills, and contextual fragment detection.

GitOrigin-RevId: 9c8b84ad54b90d174abc287ab49cc5a231e9e1aa

codex-rs/ext/skills/src/state.rs

@@ -384,6 +384,9 @@ pub(crate) struct SkillsTurnState {
     pub(crate) main_prompts_injected: bool,
 }
 
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
+pub(crate) struct HostSkillsCatalogInWorldState;
+
 #[derive(Clone, Debug, Default)]
 pub(crate) struct ExecutorSkillsStepState(pub(crate) SkillCatalog);