chore: untrack debug screenshot artifact, ignore future MCP client config

tools/debug_screenshot_gameplay.png was a one-off capture from testing the
runtime-observation MCP's screenshot tool, not something that belongs in
version control. Also ignore .mcp.json (local MCP server registration) and
future tools/debug_screenshot*.png captures.

.gitignore

@@ -62,6 +62,7 @@ tools/refs/
 
 # --- Claude Code local session/settings ---
 .claude/
+.mcp.json
 
 # --- Editor / OS ---
 .vscode/
@@ -72,3 +73,4 @@ Thumbs.db
 
 # --- Tool session cruft ---
 .playwright-mcp/
+tools/debug_screenshot*.png