docs: add comprehensive Riverpod 3.0 migration documentation and Priority 1 implementation

Priority 1 (COMPLETE):
- Add riverpod_lint and custom_lint packages
- Update analysis_options.yaml with custom_lint plugin
- Update AGENTS.md with Riverpod 3.0 best practices
- Fix unsafe ref usage in modern_chat_input.dart
- All tests passing, zero breaking changes

Priority 2 (PLANNED):
- Complete migration plan for 39 providers (RIVERPOD_PRIORITY2_PLAN.md)
- Quick reference guide (RIVERPOD_PRIORITY2_QUICKREF.md)
- Progress tracker (RIVERPOD_PRIORITY2_TRACKER.md)
- Master documentation index (RIVERPOD_MIGRATION_INDEX.md)
- Analysis and summary documents

Documentation includes:
- Step-by-step migration examples
- 6-phase implementation plan (23-33 hours)
- Testing strategies and rollback procedures
- Risk assessment and mitigation
- Timeline and resource estimates

lib/features/chat/widgets/modern_chat_input.dart

@@ -141,9 +141,8 @@ class _ModernChatInputState extends ConsumerState<ModernChatInput>
 
   @override
   void dispose() {
-    try {
-      ref.read(composerHasFocusProvider.notifier).set(false);
-    } catch (_) {}
+    // Note: Avoid using ref in dispose as per Riverpod best practices
+    // The focus state will be naturally cleared when the widget is disposed
     _controller.removeListener(_handleComposerChanged);
     _controller.dispose();
     _focusNode.dispose();