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

analysis_options.yaml

@@ -9,6 +9,10 @@
 # packages, and plugins designed to encourage good coding practices.
 include: package:flutter_lints/flutter.yaml
 
+analyzer:
+  plugins:
+    - custom_lint
+
 linter:
   # The lint rules applied to this project can be customized in the
   # section below to disable rules from the `package:flutter_lints/flutter.yaml`