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/exec-server/src/server/handler/tests.rs

@@ -292,8 +292,7 @@ async fn output_and_exit_are_retained_after_notification_receiver_closes() {
             process_id.as_str(),
             shell_argv(
                 "sleep 0.05; printf 'first\\n'; sleep 0.05; printf 'second\\n'",
-                // `cmd.exe` retains the space before `&&` in `echo first && ...`.
-                "(echo first) && ping -n 2 127.0.0.1 >NUL && (echo second)",
+                "echo first&& ping -n 2 127.0.0.1 >NUL&& echo second",
             ),
         ))
         .await