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.

example-project/project-brief.yaml

@@ -116,6 +116,17 @@ localization:
   enabled: true
   locales: ["en", "es", "fr"]
 
+# -----------------------------------------------------------------------------
+# Integrations & conventions (optional)
+# -----------------------------------------------------------------------------
+integrations:
+  mcp:
+    enabled: false   # true → .cursor/mcp.json (secrets only via env vars)
+    preset: auto       # auto | minimal
+
+conventions:
+  strict_package_imports: false
+
 # -----------------------------------------------------------------------------
 # Telemetry (Pillar 6) — local-only generation / usage log under .cursor/
 # -----------------------------------------------------------------------------