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/plugin_list.rs

@@ -30,8 +30,7 @@ use wiremock::matchers::method;
 use wiremock::matchers::path;
 use wiremock::matchers::query_param;
 
-// These tests start full app-server processes and can also run plugin startup warmers.
-const DEFAULT_TIMEOUT: Duration = Duration::from_secs(60);
+const DEFAULT_TIMEOUT: Duration = Duration::from_secs(10);
 const TEST_CURATED_PLUGIN_SHA: &str = "0123456789abcdef0123456789abcdef01234567";
 const STARTUP_REMOTE_PLUGIN_SYNC_MARKER_FILE: &str = ".tmp/app-server-remote-plugin-sync-v1";
 const ALTERNATE_MARKETPLACE_RELATIVE_PATH: &str = ".claude-plugin/marketplace.json";