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/FsCreateDirectoryResponse.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Successful response for `fs/createDirectory`.",
+  "title": "FsCreateDirectoryResponse",
+  "type": "object"
+}