app-server: define device key v2 protocol (#18428)

## Why

Clients need a stable app-server protocol surface for enrolling a local
device key, retrieving its public key, and producing a device-bound
proof.

The protocol reports `protectionClass` explicitly so clients can
distinguish hardware-backed keys from an explicitly allowed OS-protected
fallback. Signing uses a tagged `DeviceKeySignPayload` enum rather than
arbitrary bytes so each signed statement is auditable at the API
boundary.

## What changed

- Added v2 JSON-RPC methods for `device/key/create`,
`device/key/public`, and `device/key/sign`.
- Added request/response types for device-key metadata, SPKI public
keys, protection classes, and ECDSA signatures.
- Added `DeviceKeyProtectionPolicy` with hardware-only default behavior
and an explicit `allow_os_protected_nonextractable` option.
- Added the initial `remoteControlClientConnection` signing payload
variant.
- Regenerated JSON Schema and TypeScript fixtures for app-server
clients.

## Stack

This is PR 1 of 4 in the device-key app-server stack.

## Validation

- `just write-app-server-schema`
- `cargo test -p codex-app-server-protocol`

codex-rs/app-server-protocol/schema/json/v2/DeviceKeySignResponse.json

@@ -0,0 +1,33 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "DeviceKeyAlgorithm": {
+      "description": "Device-key algorithm reported at enrollment and signing boundaries.",
+      "enum": [
+        "ecdsa_p256_sha256"
+      ],
+      "type": "string"
+    }
+  },
+  "description": "ASN.1 DER signature returned by `device/key/sign`.",
+  "properties": {
+    "algorithm": {
+      "$ref": "#/definitions/DeviceKeyAlgorithm"
+    },
+    "signatureDerBase64": {
+      "description": "ECDSA signature DER encoded as base64.",
+      "type": "string"
+    },
+    "signedPayloadBase64": {
+      "description": "Exact bytes signed by the device key, encoded as base64. Verifiers must verify this byte string directly and must not reserialize `payload`.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "algorithm",
+    "signatureDerBase64",
+    "signedPayloadBase64"
+  ],
+  "title": "DeviceKeySignResponse",
+  "type": "object"
+}