feat: merge chess_tutor + OpenRouter abstraction

- Merged stefan-kp/chess_tutor into chess-project with full feature set
- Preserved Hermes agent files (agents/, .gitea/, AGENTS.md)
- Added OpenRouter abstraction layer (src/lib/openrouter.ts)
- Created ModelSelector dropdown component with 9 models
- Updated API routes (chat + opening explanation) to use OpenRouter
- Updated useTutorChat to route through OpenRouter API
- Updated onboarding and API key input for OpenRouter
- Created tasks.md with sprint 1 plan
- Updated README with new architecture details

.env.example

@@ -1,5 +1,12 @@
 # Debug Mode
 # Set to 'true' to enable debug mode which shows LLM prompts and responses
-# This is useful for debugging AI behavior
 NEXT_PUBLIC_DEBUG=false
 
+# OpenRouter API Key (optional — users can also enter in the UI)
+# Get your key at https://openrouter.ai/keys
+# OPENROUTER_API_KEY=sk-or-v1-...
+
+# Default model (user can change in the UI)
+# Options: google/gemini-2.5-flash, google/gemini-2.5-pro, anthropic/claude-sonnet-4,
+#          openai/gpt-4o, x-ai/grok-3, deepseek/deepseek-chat, meta-llama/llama-3.1-70b
+NEXT_PUBLIC_DEFAULT_MODEL=google/gemini-2.5-flash