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/rollout/src/list.rs

@@ -1082,9 +1082,6 @@ async fn read_head_summary(path: &Path, head_limit: usize) -> io::Result<HeadTai
             RolloutItem::Compacted(_) => {
                 // Not included in `head`; skip.
             }
-            RolloutItem::SessionState(_) => {
-                // Not included in `head`; skip.
-            }
             RolloutItem::EventMsg(ev) => {
                 if let EventMsg::UserMessage(user) = ev {
                     summary.saw_user_event = true;
@@ -1137,7 +1134,6 @@ pub async fn read_head_for_summary(path: &Path) -> io::Result<Vec<serde_json::Va
                     }
                 }
                 RolloutItem::Compacted(_)
-                | RolloutItem::SessionState(_)
                 | RolloutItem::TurnContext(_)
                 | RolloutItem::EventMsg(_) => {}
             }

codex-rs/rollout/src/metadata.rs

@@ -70,8 +70,7 @@ pub fn builder_from_items(
 ) -> Option<ThreadMetadataBuilder> {
     if let Some(session_meta) = items.iter().find_map(|item| match item {
         RolloutItem::SessionMeta(meta_line) => Some(meta_line),
-        RolloutItem::SessionState(_)
-        | RolloutItem::ResponseItem(_)
+        RolloutItem::ResponseItem(_)
         | RolloutItem::Compacted(_)
         | RolloutItem::TurnContext(_)
         | RolloutItem::EventMsg(_) => None,
@@ -125,8 +124,7 @@ pub async fn extract_metadata_from_rollout(
         metadata,
         memory_mode: items.iter().rev().find_map(|item| match item {
             RolloutItem::SessionMeta(meta_line) => meta_line.meta.memory_mode.clone(),
-            RolloutItem::SessionState(_)
-            | RolloutItem::ResponseItem(_)
+            RolloutItem::ResponseItem(_)
             | RolloutItem::Compacted(_)
             | RolloutItem::TurnContext(_)
             | RolloutItem::EventMsg(_) => None,

codex-rs/rollout/src/policy.rs

@@ -16,10 +16,9 @@ pub fn is_persisted_response_item(item: &RolloutItem, mode: EventPersistenceMode
         RolloutItem::ResponseItem(item) => should_persist_response_item(item),
         RolloutItem::EventMsg(ev) => should_persist_event_msg(ev, mode),
         // Persist Codex executive markers so we can analyze flows (e.g., compaction, API turns).
-        RolloutItem::Compacted(_)
-        | RolloutItem::TurnContext(_)
-        | RolloutItem::SessionMeta(_)
-        | RolloutItem::SessionState(_) => true,
+        RolloutItem::Compacted(_) | RolloutItem::TurnContext(_) | RolloutItem::SessionMeta(_) => {
+            true
+        }
     }
 }
 

codex-rs/rollout/src/recorder.rs

@@ -707,9 +707,6 @@ impl RolloutRecorder {
                     RolloutItem::Compacted(item) => {
                         items.push(RolloutItem::Compacted(item));
                     }
-                    RolloutItem::SessionState(update) => {
-                        items.push(RolloutItem::SessionState(update));
-                    }
                     RolloutItem::TurnContext(item) => {
                         items.push(RolloutItem::TurnContext(item));
                     }
@@ -1566,7 +1563,6 @@ async fn resume_candidate_matches_cwd(
         && let Some(latest_turn_context_cwd) = items.iter().rev().find_map(|item| match item {
             RolloutItem::TurnContext(turn_context) => Some(turn_context.cwd.as_path()),
             RolloutItem::SessionMeta(_)
-            | RolloutItem::SessionState(_)
             | RolloutItem::ResponseItem(_)
             | RolloutItem::Compacted(_)
             | RolloutItem::EventMsg(_) => None,