fix: fully revert agent identity runtime wiring (#18757)

## Summary

This PR fully reverts the previously merged Agent Identity runtime
integration from the old stack:
https://github.com/openai/codex/pull/17387/changes

It removes the Codex-side task lifecycle wiring, rollout/session
persistence, feature flag plumbing, lazy `auth.json` mutation,
background task auth paths, and request callsite changes introduced by
that stack.

This leaves the repo in a clean pre-AgentIdentity integration state so
the follow-up PRs can reintroduce the pieces in smaller reviewable
layers.

## Stack

1. This PR: full revert
2. https://github.com/openai/codex/pull/18871: move Agent Identity
business logic into a crate
3. https://github.com/openai/codex/pull/18785: add explicit
AgentIdentity auth mode and startup task allocation
4. https://github.com/openai/codex/pull/18811: migrate auth callsites
through AuthProvider

## Testing

Tests: targeted Rust checks, cargo-shear, Bazel lock check, and CI.

codex-rs/state/src/extract.rs

@@ -19,7 +19,6 @@ pub fn apply_rollout_item(
 ) {
     match item {
         RolloutItem::SessionMeta(meta_line) => apply_session_meta_from_item(metadata, meta_line),
-        RolloutItem::SessionState(_) => {}
         RolloutItem::TurnContext(turn_ctx) => apply_turn_context(metadata, turn_ctx),
         RolloutItem::EventMsg(event) => apply_event_msg(metadata, event),
         RolloutItem::ResponseItem(item) => apply_response_item(metadata, item),
@@ -37,10 +36,9 @@ pub fn rollout_item_affects_thread_metadata(item: &RolloutItem) -> bool {
         RolloutItem::EventMsg(
             EventMsg::TokenCount(_) | EventMsg::UserMessage(_) | EventMsg::ThreadNameUpdated(_),
         ) => true,
-        RolloutItem::SessionState(_)
-        | RolloutItem::EventMsg(_)
-        | RolloutItem::ResponseItem(_)
-        | RolloutItem::Compacted(_) => false,
+        RolloutItem::EventMsg(_) | RolloutItem::ResponseItem(_) | RolloutItem::Compacted(_) => {
+            false
+        }
     }
 }