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>

.gitignore

@@ -13,6 +13,7 @@ build/
 ## Playwright
 playwright-report/
 test-results/
+.playwright-mcp/
 
 ## Claude Code (local agent state)
 .claude/