Add workspace messages app-server API (#29001)

## Summary

- Add backend-client types and fetch support for active workspace
messages.
- Add the app-server v2 `account/workspaceMessages/read` method,
generated schemas, and README documentation.
- Delegate workspace-message eligibility to the Codex backend feature
gate; map a backend 404 to `featureEnabled: false`.

## Testing

- `just write-app-server-schema`
- `just test -p codex-backend-client`
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server workspace_messages`
- `just fix -p codex-backend-client -p codex-app-server-protocol -p
codex-app-server`
- `just fmt`

## Stack

- Base PR for #28232, which adds the TUI status-line integration.

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -2913,6 +2913,29 @@
           "title": "Account/usage/readRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/RequestId"
+            },
+            "method": {
+              "enum": [
+                "account/workspaceMessages/read"
+              ],
+              "title": "Account/workspaceMessages/readRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "type": "null"
+            }
+          },
+          "required": [
+            "id",
+            "method"
+          ],
+          "title": "Account/workspaceMessages/readRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -6705,6 +6728,28 @@
       "title": "GetAccountTokenUsageResponse",
       "type": "object"
     },
+    "GetWorkspaceMessagesResponse": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "featureEnabled": {
+          "description": "Whether the workspace-message backend route is available for this client.",
+          "type": "boolean"
+        },
+        "messages": {
+          "description": "Active workspace messages returned by the backend.",
+          "items": {
+            "$ref": "#/definitions/WorkspaceMessage"
+          },
+          "type": "array"
+        }
+      },
+      "required": [
+        "featureEnabled",
+        "messages"
+      ],
+      "title": "GetWorkspaceMessagesResponse",
+      "type": "object"
+    },
     "GitInfo": {
       "properties": {
         "branch": {
@@ -18343,6 +18388,49 @@
       "title": "WindowsWorldWritableWarningNotification",
       "type": "object"
     },
+    "WorkspaceMessage": {
+      "properties": {
+        "archivedAt": {
+          "description": "Unix timestamp (in seconds) when the message was archived.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "createdAt": {
+          "description": "Unix timestamp (in seconds) when the message was created.",
+          "format": "int64",
+          "type": [
+            "integer",
+            "null"
+          ]
+        },
+        "messageBody": {
+          "type": "string"
+        },
+        "messageId": {
+          "type": "string"
+        },
+        "messageType": {
+          "$ref": "#/definitions/WorkspaceMessageType"
+        }
+      },
+      "required": [
+        "messageBody",
+        "messageId",
+        "messageType"
+      ],
+      "type": "object"
+    },
+    "WorkspaceMessageType": {
+      "enum": [
+        "headline",
+        "announcement",
+        "unknown"
+      ],
+      "type": "string"
+    },
     "WriteStatus": {
       "enum": [
         "ok",