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/cloud-tasks-client/src/http.rs

@@ -37,11 +37,6 @@ impl HttpClient {
         self
     }
 
-    pub fn with_authorization_header_value(mut self, value: impl Into<String>) -> Self {
-        self.backend = self.backend.clone().with_authorization_header_value(value);
-        self
-    }
-
     pub fn with_user_agent(mut self, ua: impl Into<String>) -> Self {
         self.backend = self.backend.clone().with_user_agent(ua);
         self
@@ -52,11 +47,6 @@ impl HttpClient {
         self
     }
 
-    pub fn with_fedramp_routing_header(mut self) -> Self {
-        self.backend = self.backend.clone().with_fedramp_routing_header();
-        self
-    }
-
     fn tasks_api(&self) -> api::Tasks<'_> {
         api::Tasks::new(self)
     }