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.toml

@@ -2,6 +2,7 @@
 members = [
     "aws-auth",
     "analytics",
+    "agent-identity",
     "backend-client",
     "ansi-escape",
     "async-utils",
@@ -235,6 +236,7 @@ clap_complete = "4"
 color-eyre = "0.6.3"
 constant_time_eq = "0.3.1"
 crossbeam-channel = "0.5.15"
+crypto_box = { version = "0.9.1", features = ["seal"] }
 crossterm = "0.28.1"
 csv = "1.3.1"
 ctor = "0.6.3"
@@ -245,6 +247,7 @@ 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"