Add configurable session persistence

[brunoamancio]

• add a CLI parser so --session-persist, --no-session-persist, and --session-persist=<path> (plus CODEX_SESSION_PERSIST / CODEX_SESSION_DIR) can be handled before startup
• introduce a SessionStore manifest that records rollout paths, MCP config, and model/mode settings for each session
• persist metadata on new sessions, resume via rollouts in /session/load, and keep the manifest in sync when models or modes change
• expose the new capability in the agent capabilities handshake and update the README with usage instructions

[benbrandt]

@brunoamancio thanks for taking the time on this. I'll have to dedicate some time to look at the implementation in detail, since this is adding our own persistence I'll want to think through this with keeping future list and restore features in mind

[brunoamancio]

@benbrandt np. I'm working (non-commercially) on a plugin for Jetbrains IDEs that will use acp for ai agents and the lack of this functionality was holding me back 😥 I can link it here if/when I publish it, if that's allowed.
Anyway, take your time thinking on this issue and solution. Also let me know if you have any feedback or insights.