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

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

codex-rs/Cargo.toml

@@ -25,6 +25,7 @@ members = [
     "shell-escalation",
     "skills",
     "core",
+    "core-skills",
     "hooks",
     "instructions",
     "secrets",
@@ -119,6 +120,7 @@ codex-cloud-requirements = { path = "cloud-requirements" }
 codex-connectors = { path = "connectors" }
 codex-config = { path = "config" }
 codex-core = { path = "core" }
+codex-core-skills = { path = "core-skills" }
 codex-exec = { path = "exec" }
 codex-exec-server = { path = "exec-server" }
 codex-execpolicy = { path = "execpolicy" }