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Keep explicit invocation telemetry, unreadable-skill warnings, plugin prompt ordering, and suppression of host prompts superseded by provider prompts.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/selection.rs

@@ -1,7 +1,7 @@
 use std::collections::HashSet;
 
-use codex_core_skills::injection::extract_tool_mentions;
 use codex_protocol::user_input::UserInput;
+use codex_skills::extract_tool_mentions;
 
 use crate::catalog::SkillAuthority;
 use crate::catalog::SkillCatalog;