app-server: add v2 filesystem APIs (#14245)

Add a protocol-level filesystem surface to the v2 app-server so Codex
clients can read and write files, inspect directories, and subscribe to
path changes without relying on host-specific helpers.

High-level changes:
- define the new v2 fs/readFile, fs/writeFile, fs/createDirectory,
fs/getMetadata, fs/readDirectory, fs/remove, fs/copy RPCs
- implement the app-server handlers, including absolute-path validation,
base64 file payloads, recursive copy/remove semantics
- document the API, regenerate protocol schemas/types, and add
end-to-end tests for filesystem operations, copy edge cases

Testing plan:
- validate protocol serialization and generated schema output for the
new fs request, response, and notification types
- run app-server integration coverage for file and directory CRUD paths,
metadata/readDirectory responses, copy failure modes, and absolute-path
validation

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

@@ -634,6 +634,164 @@
       ],
       "type": "object"
     },
+    "FsCopyParams": {
+      "description": "Copy a file or directory tree on the host filesystem.",
+      "properties": {
+        "destinationPath": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute destination path."
+        },
+        "recursive": {
+          "description": "Required for directory copies; ignored for file copies.",
+          "type": "boolean"
+        },
+        "sourcePath": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute source path."
+        }
+      },
+      "required": [
+        "destinationPath",
+        "sourcePath"
+      ],
+      "type": "object"
+    },
+    "FsCreateDirectoryParams": {
+      "description": "Create a directory on the host filesystem.",
+      "properties": {
+        "path": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute directory path to create."
+        },
+        "recursive": {
+          "description": "Whether parent directories should also be created. Defaults to `true`.",
+          "type": [
+            "boolean",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "path"
+      ],
+      "type": "object"
+    },
+    "FsGetMetadataParams": {
+      "description": "Request metadata for an absolute path.",
+      "properties": {
+        "path": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute path to inspect."
+        }
+      },
+      "required": [
+        "path"
+      ],
+      "type": "object"
+    },
+    "FsReadDirectoryParams": {
+      "description": "List direct child names for a directory.",
+      "properties": {
+        "path": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute directory path to read."
+        }
+      },
+      "required": [
+        "path"
+      ],
+      "type": "object"
+    },
+    "FsReadFileParams": {
+      "description": "Read a file from the host filesystem.",
+      "properties": {
+        "path": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute path to read."
+        }
+      },
+      "required": [
+        "path"
+      ],
+      "type": "object"
+    },
+    "FsRemoveParams": {
+      "description": "Remove a file or directory tree from the host filesystem.",
+      "properties": {
+        "force": {
+          "description": "Whether missing paths should be ignored. Defaults to `true`.",
+          "type": [
+            "boolean",
+            "null"
+          ]
+        },
+        "path": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute path to remove."
+        },
+        "recursive": {
+          "description": "Whether directory removal should recurse. Defaults to `true`.",
+          "type": [
+            "boolean",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "path"
+      ],
+      "type": "object"
+    },
+    "FsWriteFileParams": {
+      "description": "Write a file on the host filesystem.",
+      "properties": {
+        "dataBase64": {
+          "description": "File contents encoded as base64.",
+          "type": "string"
+        },
+        "path": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/AbsolutePathBuf"
+            }
+          ],
+          "description": "Absolute path to write."
+        }
+      },
+      "required": [
+        "dataBase64",
+        "path"
+      ],
+      "type": "object"
+    },
     "FunctionCallOutputBody": {
       "anyOf": [
         {
@@ -3670,6 +3828,174 @@
       "title": "App/listRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/readFile"
+          ],
+          "title": "Fs/readFileRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsReadFileParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/readFileRequest",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/writeFile"
+          ],
+          "title": "Fs/writeFileRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsWriteFileParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/writeFileRequest",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/createDirectory"
+          ],
+          "title": "Fs/createDirectoryRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsCreateDirectoryParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/createDirectoryRequest",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/getMetadata"
+          ],
+          "title": "Fs/getMetadataRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsGetMetadataParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/getMetadataRequest",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/readDirectory"
+          ],
+          "title": "Fs/readDirectoryRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsReadDirectoryParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/readDirectoryRequest",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/remove"
+          ],
+          "title": "Fs/removeRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsRemoveParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/removeRequest",
+      "type": "object"
+    },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "fs/copy"
+          ],
+          "title": "Fs/copyRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/FsCopyParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Fs/copyRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {