Add session loading support to Codex (#1602)

## Summary
- extend rollout format to store all session data in JSON
- add resume/write helpers for rollouts
- track session state after each conversation
- support `LoadSession` op to resume a previous rollout
- allow starting Codex with an existing session via
`experimental_resume` config variable

We need a way later for exploring the available sessions in a user
friendly way.

## Testing
- `cargo test --no-run` *(fails: `cargo: command not found`)*

------
https://chatgpt.com/codex/tasks/task_i_68792a29dd5c832190bf6930d3466fba

This video is outdated. you should use `-c experimental_resume:<full
path>` instead of `--resume <full path>`


https://github.com/user-attachments/assets/7a9975c7-aa04-4f4e-899a-9e87defd947a

codex-rs/core/src/protocol.rs

@@ -69,6 +69,10 @@ pub enum Op {
         /// `ConfigureSession` operation so that the business-logic layer can
         /// operate deterministically.
         cwd: std::path::PathBuf,
+
+        /// Path to a rollout file to resume from.
+        #[serde(skip_serializing_if = "Option::is_none")]
+        resume_path: Option<std::path::PathBuf>,
     },
 
     /// Abort current task.