fix(ci): unbreak build for the source-less TypeScript SDK

The SDK scaffolding landed without a src/ directory, so all three CI
steps fail on main:

  - npm run build -> TS18003: No inputs were found in config file
  - npm test      -> No test files found, exiting with code 1
  - npm run lint  -> No files matching the pattern "src/" were found

Add a placeholder src/index.ts so tsc and eslint have inputs, and pass
--passWithNoTests to vitest so the suite stays green until real tests
land. No API surface is introduced; that waits on the extension spec.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DsTktbxWN5RQsEE1EX6wGm

sdk/typescript/package.json

@@ -10,7 +10,7 @@
     ],
     "scripts": {
         "build": "tsc",
-        "test": "vitest run",
+        "test": "vitest run --passWithNoTests",
         "test:watch": "vitest",
         "lint": "eslint src/ && prettier --check .",
         "format": "prettier --write ."

sdk/typescript/src/index.ts

@@ -0,0 +1,5 @@
+// Entry point for the MCP Grouping Extension SDK.
+// The public API is added as the grouping extension spec takes shape; this
+// placeholder keeps the package buildable in the meantime.
+
+export {};