Register agent tasks behind use_agent_identity (#17387)

## Summary

Stack PR3 for feature-gated agent identity support.

This PR adds per-thread agent task registration behind
`features.use_agent_identity`. Tasks are minted on the first real user
turn and cached in thread runtime state for later turns.

## Stack

- PR1: https://github.com/openai/codex/pull/17385 - add
`features.use_agent_identity`
- PR2: https://github.com/openai/codex/pull/17386 - register agent
identities when enabled
- PR3: https://github.com/openai/codex/pull/17387 - this PR, original
task registration slice
- PR3.1: https://github.com/openai/codex/pull/17978 - persist and
prewarm registered tasks per thread
- PR4: https://github.com/openai/codex/pull/17980 - use `AgentAssertion`
downstream when enabled

## Validation

Covered as part of the local stack validation pass:

- `just fmt`
- `cargo test -p codex-core --lib agent_identity`
- `cargo test -p codex-core --lib agent_assertion`
- `cargo test -p codex-core --lib websocket_agent_task`
- `cargo test -p codex-api api_bridge`
- `cargo build -p codex-cli --bin codex`

## Notes

The full local app-server E2E path is still being debugged after PR
creation. The current branch stack is directionally ready for review
while that follow-up continues.

codex-rs/Cargo.lock

@@ -940,6 +940,15 @@ 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"
@@ -1960,6 +1969,7 @@ dependencies = [
  "codex-windows-sandbox",
  "core-foundation 0.9.4",
  "core_test_support",
+ "crypto_box",
  "csv",
  "ctor 0.6.3",
  "dirs",
@@ -1990,6 +2000,7 @@ dependencies = [
  "serde_json",
  "serial_test",
  "sha1",
+ "sha2",
  "shlex",
  "similar",
  "tempfile",
@@ -3656,9 +3667,40 @@ 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"
@@ -4948,6 +4990,7 @@ checksum = "85649ca51fd72272d7821adaf274ad91c288277713d9c18820d8499a7ff69e9a"
 dependencies = [
  "typenum",
  "version_check",
+ "zeroize",
 ]
 
 [[package]]