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.lock

@@ -946,15 +946,6 @@ dependencies = [
  "serde_core",
 ]
 
-[[package]]
-name = "blake2"
-version = "0.10.6"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "46502ad458c9a52b69d4d4d32775c788b7a1b85e8bc9d482d92250fc0e3f8efe"
-dependencies = [
- "digest",
-]
-
 [[package]]
 name = "block-buffer"
 version = "0.10.4"
@@ -1946,7 +1937,6 @@ dependencies = [
  "codex-api",
  "codex-app-server-protocol",
  "codex-apply-patch",
- "codex-arg0",
  "codex-async-utils",
  "codex-code-mode",
  "codex-config",
@@ -1996,12 +1986,10 @@ dependencies = [
  "codex-windows-sandbox",
  "core-foundation 0.9.4",
  "core_test_support",
- "crypto_box",
  "csv",
  "ctor 0.6.3",
  "dirs",
  "dunce",
- "ed25519-dalek",
  "env-flags",
  "eventsource-stream",
  "futures",
@@ -2027,7 +2015,6 @@ dependencies = [
  "serde_json",
  "serial_test",
  "sha1",
- "sha2",
  "shlex",
  "similar",
  "tempfile",
@@ -2417,8 +2404,6 @@ dependencies = [
  "codex-terminal-detection",
  "codex-utils-template",
  "core_test_support",
- "crypto_box",
- "ed25519-dalek",
  "keyring",
  "once_cell",
  "os_info",
@@ -3753,40 +3738,9 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "78c8292055d1c1df0cce5d180393dc8cce0abec0a7102adb6c7b1eef6016d60a"
 dependencies = [
  "generic-array",
- "rand_core 0.6.4",
  "typenum",
 ]
 
-[[package]]
-name = "crypto_box"
-version = "0.9.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "16182b4f39a82ec8a6851155cc4c0cda3065bb1db33651726a29e1951de0f009"
-dependencies = [
- "aead",
- "blake2",
- "crypto_secretbox",
- "curve25519-dalek",
- "salsa20",
- "subtle",
- "zeroize",
-]
-
-[[package]]
-name = "crypto_secretbox"
-version = "0.1.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b9d6cf87adf719ddf43a805e92c6870a531aedda35ff640442cbaf8674e141e1"
-dependencies = [
- "aead",
- "cipher",
- "generic-array",
- "poly1305",
- "salsa20",
- "subtle",
- "zeroize",
-]
-
 [[package]]
 name = "csv"
 version = "1.4.0"
@@ -3843,7 +3797,6 @@ dependencies = [
  "cfg-if",
  "cpufeatures",
  "curve25519-dalek-derive",
- "digest",
  "fiat-crypto",
  "rustc_version",
  "subtle",
@@ -4464,30 +4417,6 @@ dependencies = [
  "spki",
 ]
 
-[[package]]
-name = "ed25519"
-version = "2.2.3"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "115531babc129696a58c64a4fef0a8bf9e9698629fb97e9e40767d235cfbcd53"
-dependencies = [
- "pkcs8",
- "signature",
-]
-
-[[package]]
-name = "ed25519-dalek"
-version = "2.2.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "70e796c081cee67dc755e1a36a0a172b897fab85fc3f6bc48307991f64e4eca9"
-dependencies = [
- "curve25519-dalek",
- "ed25519",
- "serde",
- "sha2",
- "subtle",
- "zeroize",
-]
-
 [[package]]
 name = "either"
 version = "1.15.0"