feat: overhaul CLI with new AI commands and mole-style menu (#6)

Major CLI redesign introducing a mole-style interactive menu with grouped categories. Added new AI-powered subcommands: explain, review, changelog, and suggest. Includes streaming output, pipe support, and updated brand logo. Refactored codebase into command modules for maintainability.

Reviewed-on: #6

package.json

@@ -1,7 +1,7 @@
 {
   "name": "gai",
-  "version": "0.1.2",
-  "description": "AI-powered git commit message generator",
+  "version": "0.1.3",
+  "description": "AI-powered git helper — commit messages, PRs, code review, changelogs, and more",
   "module": "index.ts",
   "type": "module",
   "bin": {