Encrypt multi-agent v2 message payloads (#26210)

## Why

Multi-agent v2 currently routes agent instructions through normal tool
arguments and inter-agent context. That means the parent model can emit
plaintext task text, Codex can persist it in history/rollouts, and the
recipient can receive it as ordinary assistant-message JSON.

This changes the v2 path so agent instructions stay encrypted between
model calls: Responses encrypts the `message` argument returned by the
model, Codex forwards only that ciphertext, and Responses decrypts it
internally for the recipient model.

## What changed

- Mark the v2 `message` parameter as encrypted for `spawn_agent`,
`send_message`, and `followup_task`.
- Treat multi-agent v2 tool `message` values as ciphertext
unconditionally.
- Store v2 inter-agent task text in
`InterAgentCommunication.encrypted_content` with empty plaintext
`content`.
- Convert encrypted inter-agent communications into the Responses
`agent_message` input item before sending the child request.
- Preserve `agent_message` items across history, rollout, compaction,
telemetry, and app-server schema paths.
- Leave multi-agent v1 unchanged.

## Message shape

The model still calls the v2 tools with a `message` argument, but that
value is now ciphertext:

```json
{
  "name": "spawn_agent",
  "arguments": {
    "task_name": "worker",
    "message": "<ciphertext>"
  }
}
```

Codex stores the task as encrypted inter-agent communication:

```json
{
  "author": "/root",
  "recipient": "/root/worker",
  "content": "",
  "encrypted_content": "<ciphertext>",
  "trigger_turn": true
}
```

When Codex builds the recipient request, it forwards the ciphertext
using the new Responses input item:

```json
{
  "type": "agent_message",
  "author": "/root",
  "recipient": "/root/worker",
  "content": [
    {
      "type": "encrypted_content",
      "encrypted_content": "<ciphertext>"
    }
  ]
}
```

Responses decrypts that item internally for the recipient model.

## Context impact

- Parent context no longer carries plaintext v2 agent task instructions
from these tool arguments.
- Codex rollout/history stores ciphertext for v2 agent instructions.
- Recipient requests receive an `agent_message` item instead of
assistant commentary JSON for encrypted task delivery.
- Plaintext completion/status notifications are still plaintext because
they are Codex-generated status messages, not encrypted model tool
arguments.

## Validation

- `just test -p codex-tools`
- `just test -p codex-protocol`
- `just test -p codex-rollout`
- `just test -p codex-rollout-trace`
- `just test -p codex-otel`
- `just write-app-server-schema`

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -34,6 +34,30 @@
       ],
       "type": "string"
     },
+    "AgentMessageInputContent": {
+      "oneOf": [
+        {
+          "properties": {
+            "encrypted_content": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "encrypted_content"
+              ],
+              "title": "EncryptedContentAgentMessageInputContentType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "encrypted_content",
+            "type"
+          ],
+          "title": "EncryptedContentAgentMessageInputContent",
+          "type": "object"
+        }
+      ]
+    },
     "ApprovalsReviewer": {
       "description": "Configures who approval requests are routed to for review. Examples include sandbox escapes, blocked network access, MCP approval prompts, and ARC escalations. Defaults to `user`. `auto_review` uses a carefully prompted subagent to gather relevant context and apply a risk-based decision framework before approving or denying the request. The legacy value `guardian_subagent` is accepted for compatibility.",
       "enum": [
@@ -2120,6 +2144,37 @@
           "title": "MessageResponseItem",
           "type": "object"
         },
+        {
+          "properties": {
+            "author": {
+              "type": "string"
+            },
+            "content": {
+              "items": {
+                "$ref": "#/definitions/AgentMessageInputContent"
+              },
+              "type": "array"
+            },
+            "recipient": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "agent_message"
+              ],
+              "title": "AgentMessageResponseItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "author",
+            "content",
+            "recipient",
+            "type"
+          ],
+          "title": "AgentMessageResponseItem",
+          "type": "object"
+        },
         {
           "properties": {
             "content": {