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/realtime_conversation.rs

@@ -72,7 +72,6 @@ use wiremock::matchers::path;
 use wiremock::matchers::path_regex;
 
 const DEFAULT_TIMEOUT: Duration = Duration::from_secs(10);
-const DELEGATED_SHELL_TOOL_TIMEOUT_MS: u64 = 30_000;
 const STARTUP_CONTEXT_HEADER: &str = "Startup context from Codex.";
 const V2_STEERING_ACKNOWLEDGEMENT: &str =
     "This was sent to steer the previous background agent task.";
@@ -1782,9 +1781,7 @@ async fn webrtc_v2_tool_call_delegated_turn_can_execute_shell_tool() -> Result<(
         create_shell_command_sse_response(
             realtime_tool_ok_command(),
             /*workdir*/ None,
-            // Windows CI can spend several seconds starting the nested PowerShell command. This
-            // test verifies delegated shell-tool plumbing, not timeout enforcement.
-            Some(DELEGATED_SHELL_TOOL_TIMEOUT_MS),
+            Some(5000),
             "shell_call",
         )?,
         create_final_assistant_message_sse_response("shell tool finished")?,