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/features/src/tests.rs

@@ -225,12 +225,6 @@ fn remote_control_is_under_development() {
     assert_eq!(Feature::RemoteControl.default_enabled(), false);
 }
 
-#[test]
-fn use_agent_identity_is_under_development() {
-    assert_eq!(Feature::UseAgentIdentity.stage(), Stage::UnderDevelopment);
-    assert_eq!(Feature::UseAgentIdentity.default_enabled(), false);
-}
-
 #[test]
 fn workspace_dependencies_is_stable_and_enabled_by_default() {
     assert_eq!(Feature::WorkspaceDependencies.stage(), Stage::Stable);