Extract codex-core-skills crate (#15749)

## Summary
- move skill loading and management into codex-core-skills
- leave codex-core with the thin integration layer and shared wiring

## Testing
- CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core-skills/src/system.rs

@@ -0,0 +1,9 @@
+pub(crate) use codex_skills::install_system_skills;
+pub(crate) use codex_skills::system_cache_root_dir;
+
+use std::path::Path;
+
+pub(crate) fn uninstall_system_skills(codex_home: &Path) {
+    let system_skills_dir = system_cache_root_dir(codex_home);
+    let _ = std::fs::remove_dir_all(&system_skills_dir);
+}