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/Cargo.toml

@@ -229,7 +229,6 @@ color-eyre = "0.6.3"
 constant_time_eq = "0.3.1"
 crossbeam-channel = "0.5.15"
 crossterm = "0.28.1"
-crypto_box = { version = "0.9.1", features = ["seal"] }
 csv = "1.3.1"
 ctor = "0.6.3"
 deno_core_icudata = "0.77.0"
@@ -239,7 +238,6 @@ dirs = "6"
 dns-lookup = "3.0.1"
 dotenvy = "0.15.7"
 dunce = "1.0.4"
-ed25519-dalek = { version = "2.2.0", features = ["pkcs8"] }
 encoding_rs = "0.8.35"
 env-flags = "0.1.1"
 env_logger = "0.11.9"