app-server: add v2 filesystem APIs

Add the v2 app-server protocol and implementation for filesystem reads, writes, directory operations, metadata, copying, and removal. This also updates the schema fixtures, app-server documentation, and the v2 integration coverage for the filesystem API surface.

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

@@ -0,0 +1,15 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Base64-encoded file contents returned by `fs/readFile`.",
+  "properties": {
+    "dataBase64": {
+      "description": "File contents encoded as base64.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "dataBase64"
+  ],
+  "title": "FsReadFileResponse",
+  "type": "object"
+}