Migrate tui to use UserTurn (#9497)

- `tui/` and `tui2/` submit `Op::UserTurn` and own full turn context
(cwd/approval/sandbox/model/etc.).
- `Op::UserInput` is documented as legacy in `codex-protocol` (doc-only;
no `#[deprecated]` to avoid `-D warnings` fallout).
- Remove obsolete `#[allow(deprecated)]` and the unused `ConversationId`
alias/re-export.

codex-rs/protocol/src/lib.rs

@@ -1,7 +1,5 @@
 pub mod account;
 mod thread_id;
-#[allow(deprecated)]
-pub use thread_id::ConversationId;
 pub use thread_id::ThreadId;
 pub mod approvals;
 pub mod config_types;