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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "gims",
-  "version": "0.6.1",
+  "version": "0.6.2",
   "description": "Git Made Simple – AI‑powered git helper using Gemini / OpenAI",
   "author": "S41R4J",
   "license": "MIT",