feat: update to latest AI models and simplify README

- Update AI models to latest versions:
  - Gemini: gemini-2.5-flash
  - OpenAI: gpt-5
  - Groq: groq/compound
- Drastically simplify README (90% shorter)
- Focus on quick start and essential commands
- Update version to 0.6.2

bin/lib/ai/providers.js

@@ -28,9 +28,9 @@ class AIProviderManager {
 
   getDefaultModel(provider) {
     const defaults = {
-      'gemini': 'gemini-2.0-flash-exp',  // Latest and fastest
-      'openai': 'gpt-4o-mini',           // Cost-effective and fast
-      'groq': 'llama-3.1-8b-instant'    // Fast inference
+      'gemini': 'gemini-2.5-flash',     // Latest and fastest
+      'openai': 'gpt-5',                // Latest GPT model
+      'groq': 'groq/compound'           // Latest Groq model
     };
     return defaults[provider] || '';
   }