Extract agent role loading into a dedicated crate (#40487)

## What changed

- Add `codex-agent-roles` for agent role configuration types, parsing,
  discovery, validation, and layered loading.
- Update `codex-core` to consume the new crate's public role configuration and
  parsing APIs.

GitOrigin-RevId: 5a963a219a581b2848dae6fd071aa31a0b5ff22c

codex-rs/core/Cargo.toml

@@ -25,6 +25,7 @@ chrono = { workspace = true, features = ["serde"] }
 clap = { workspace = true, features = ["derive"] }
 codex-analytics = { workspace = true }
 codex-agent-graph-store = { workspace = true }
+codex-agent-roles = { workspace = true }
 codex-api = { workspace = true }
 codex-app-server-protocol = { workspace = true }
 codex-apply-patch = { workspace = true }