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/core/config.schema.json

@@ -521,9 +521,6 @@
             "unified_exec": {
               "type": "boolean"
             },
-            "use_agent_identity": {
-              "type": "boolean"
-            },
             "use_legacy_landlock": {
               "type": "boolean"
             },
@@ -2520,9 +2517,6 @@
         "unified_exec": {
           "type": "boolean"
         },
-        "use_agent_identity": {
-          "type": "boolean"
-        },
         "use_legacy_landlock": {
           "type": "boolean"
         },