Add project .mcp.json to enable Chromium sandbox for Playwright MCP

Playwright MCP v0.0.70 defaults to launching Chrome with --no-sandbox
because its sandbox-inference code in playwright-core mcp/config.js
only sets the default when browser.browserName is explicitly 'chromium';
the MCP leaves it undefined, so the default-to-true branch is skipped
and Playwright core pushes --no-sandbox into chromeArguments.

Chrome surfaces this as a yellow warning bar on every Playwright-driven
tab. Setting PLAYWRIGHT_MCP_SANDBOX=true via the MCP server env forces
chromiumSandbox=true, which skips the --no-sandbox push and removes the
warning. The override is scoped to this project; the plugin's own
.mcp.json is unchanged.

.mcp.json

@@ -0,0 +1,11 @@
+{
+  "mcpServers": {
+    "playwright": {
+      "command": "npx",
+      "args": ["@playwright/mcp@latest"],
+      "env": {
+        "PLAYWRIGHT_MCP_SANDBOX": "true"
+      }
+    }
+  }
+}