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/app-server/tests/suite/v2/initialize.rs

@@ -24,9 +24,7 @@ use std::time::Duration;
 use tempfile::TempDir;
 use tokio::time::timeout;
 
-// This covers debug app-server process startup before the first JSON-RPC response,
-// not expected steady-state request latency.
-const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(20);
+const DEFAULT_READ_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(10);
 
 #[tokio::test]
 async fn initialize_uses_client_info_name_as_originator() -> Result<()> {