refactor: add agent identity crate (#18871)

## Summary

This PR adds `codex-agent-identity` as an isolated crate for Agent
Identity business logic.

The crate owns:
- AgentAssertion construction.
- Agent task registration.
- private-key assertion signing.
- bounded blocking HTTP for task registration.

It does not wire AgentIdentity into `auth.json`, `AuthManager`, rollout
state, or request callsites. That integration happens in later PRs.

Reference old stack: https://github.com/openai/codex/pull/17387/changes

## Stack

1. https://github.com/openai/codex/pull/18757: full revert
2. This PR: isolated Agent Identity crate
3. https://github.com/openai/codex/pull/18785: explicit AgentIdentity
auth mode and startup task allocation
4. https://github.com/openai/codex/pull/18811: migrate Codex backend
auth callsites through AuthProvider
5. https://github.com/openai/codex/pull/18904: accept AgentIdentity JWTs
and load `CODEX_AGENT_IDENTITY`

## Testing

Tests: targeted Rust checks, cargo-shear, Bazel lock check, and CI.

codex-rs/Cargo.lock

@@ -1282,6 +1282,15 @@ 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"
@@ -1727,6 +1736,23 @@ dependencies = [
  "unicode-width 0.2.1",
 ]
 
+[[package]]
+name = "codex-agent-identity"
+version = "0.0.0"
+dependencies = [
+ "anyhow",
+ "base64 0.22.1",
+ "chrono",
+ "codex-protocol",
+ "crypto_box",
+ "ed25519-dalek",
+ "pretty_assertions",
+ "rand 0.9.3",
+ "serde",
+ "serde_json",
+ "sha2",
+]
+
 [[package]]
 name = "codex-analytics"
 version = "0.0.0"
@@ -4144,9 +4170,40 @@ 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"
@@ -4203,6 +4260,7 @@ dependencies = [
  "cfg-if",
  "cpufeatures",
  "curve25519-dalek-derive",
+ "digest",
  "fiat-crypto",
  "rustc_version",
  "subtle",
@@ -4832,6 +4890,30 @@ 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"