feat(flutter-cursor-templates): introduce MCP integration and conventions in project brief

- Added optional MCP integration settings in project-brief.yaml, allowing for environment-based server configurations.
- Introduced conventions for strict package imports to enhance code organization and maintainability.
- Updated brief schema to validate new MCP properties and ensure correct usage.
- Implemented MCP JSON builder to generate .cursor/mcp.json based on project brief settings.
- Enhanced resolver to include MCP configuration in generated files when enabled.

This update improves integration capabilities and enforces coding standards across the project.

flutter-cursor-templates/templates/rules/universal/project-context.mdc.tmpl

@@ -1,6 +1,7 @@
 ---
-description: "Project context for {{PROJECT_NAME}} — always applied"
-alwaysApply: true
+description: "Stack summary and product context for {{PROJECT_NAME}}"
+globs: ["project-brief.yaml", ".cursor/**/*.md", ".cursor/**/*.mdc", "pubspec.yaml"]
+alwaysApply: false
 ---
 
 # Project Context — {{PROJECT_NAME}}