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/config/Cargo.toml

@@ -14,6 +14,7 @@ codex-protocol = { workspace = true }
 codex-utils-absolute-path = { workspace = true }
 futures = { workspace = true, features = ["alloc", "std"] }
 multimap = { workspace = true }
+schemars = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 serde_path_to_error = { workspace = true }