refactor(cli): extract interactive menu into reusable module (#3)

Extract duplicate menu rendering logic from `index.ts` into a new `src/menu.ts` module, providing generic `selectOne` and `selectMany` functions. This reduces code duplication, improves maintainability, and adds consistent UI controls display across the commit flow and platform selection.

Reviewed-on: #3

README.md

@@ -60,10 +60,12 @@ gai --version      Show version
 $ gai
 
   gai
+  Choose a workflow
+  ↑/↓ navigate · enter/space select · ctrl+c cancel
 
-  ↑/↓ navigate, space/enter select
-
-  ❯ ◉ commit          Generate AI commit message
+  ❯ ● commit  Generate AI commit message
+    ○ pr      Create a PR with AI-generated title
+    ○ config  Configure API settings
 ```
 
 ### Commit Flow
@@ -74,14 +76,13 @@ $ gai commit
   Staged files (will be included):
     ✓ src/git.ts (modified)
 
-  Unstaged files:
-    1. src/ai.ts (modified)
-    2. src/newfile.ts (new)
-
   Select files to stage:
-    ❯ ◉ Select all
-      ○ src/ai.ts (modified)
-      ◉ src/newfile.ts (new)
+  2 unstaged files available
+  ↑/↓ navigate · space toggle · enter confirm · ctrl+c cancel
+
+  ❯ □ Select all
+    □ src/ai.ts      modified
+    ■ src/newfile.ts new
 
   Generating commit message...