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/auth/mod.rs

@@ -1,4 +1,3 @@
-mod agent_assertion;
 pub mod default_client;
 pub mod error;
 mod storage;
@@ -8,7 +7,6 @@ mod external_bearer;
 mod manager;
 mod revoke;
 
-pub use agent_assertion::AgentTaskAuthorizationTarget;
 pub use error::RefreshTokenFailedError;
 pub use error::RefreshTokenFailedReason;
 pub use manager::*;