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/login/src/lib.rs

@@ -1,4 +1,3 @@
-pub mod agent_identity;
 pub mod auth;
 pub mod auth_env_telemetry;
 pub mod token_data;
@@ -18,10 +17,6 @@ pub use server::ServerOptions;
 pub use server::ShutdownHandle;
 pub use server::run_login_server;
 
-pub use agent_identity::BackgroundAgentTaskAuthMode;
-pub use agent_identity::cached_background_agent_task_authorization_header_value;
-pub use auth::AgentIdentityAuthRecord;
-pub use auth::AgentTaskAuthorizationTarget;
 pub use auth::AuthConfig;
 pub use auth::AuthDotJson;
 pub use auth::AuthManager;