Remove the codex-core-skills crate (#37505)

## What changed

- Move `SkillLoadOutcome` and its implicit-path indexing into
  `codex-skills-extension`.
- Import shared skill types directly from `codex-skills` and keep the skill
  prompt size limit with the extension renderer that uses it.
- Remove `codex-core-skills` from the Cargo workspace and dependent crates.

GitOrigin-RevId: 4e9e84909fa2f692bcc94af990bb4671affd776f

codex-rs/ext/skills/src/provider/host.rs

@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 
-use codex_core_skills::SkillLoadOutcome;
+use crate::SkillLoadOutcome;
 use codex_skills::SkillMetadata;
 
 use crate::catalog::SkillAuthority;