Add Playwright MCP server config and ignore its scratch dir

Checks in `.mcp.json` so the Playwright MCP server is available to
all contributors out of the box, and ignores `.playwright-mcp/` where
the server stashes per-session snapshots and console logs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.mcp.json

@@ -0,0 +1,13 @@
+{
+  "mcpServers": {
+    "playwright": {
+      "type": "stdio",
+      "command": "npx.cmd",
+      "args": [
+        "-y",
+        "@playwright/mcp@latest"
+      ],
+      "env": {}
+    }
+  }
+}