chore: ignore the agent-generated .mcp.json

The harness writes it into the checkout next to the rendered .agents/ and .claude/ install targets, so it belongs to the same ignored group.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitignore

@@ -15,6 +15,7 @@ npm-debug.log*
 # Rendered skill install targets
 .agents/
 .claude/
+.mcp.json
 
 # Editor / OS
 .DS_Store