feat(cli): add stats compare and export subcommands

- Restructure stats command to support subcommands:
  - 'dev stats' or 'dev stats show' - Show current stats (default)
  - 'dev stats compare <before> <after>' - Compare two stat snapshots
  - 'dev stats export [--format json|csv|markdown]' - Export stats

Features:
- Compare shows overall changes, trend analysis, and per-language/component diffs
- Export supports JSON, CSV, and Markdown table formats
- Rich visual comparison with color-coded changes and percentages
- Helper function for human-readable duration formatting

Benefits:
- Users can track codebase evolution over time
- Dashboard-ready data export for visualization
- Clean CLI UX with consistent formatting
- Maintains backward compatibility (stats show is default)

All 736 tests passing (including updated CLI structure tests).

Author: prosdev

packages/cli/src/cli.test.ts

@@ -58,11 +58,16 @@ describe('CLI Structure', () => {
       expect(jsonOption).toBeDefined();
     });
 
-    it('stats command should have json option', () => {
-      const options = statsCommand.options;
-      const jsonOption = options.find((opt) => opt.long === '--json');
+    it('stats command should have show subcommand with json option', () => {
+      const subcommands = statsCommand.commands;
+      const showCommand = subcommands.find((cmd) => cmd.name() === 'show');
 
-      expect(jsonOption).toBeDefined();
+      expect(showCommand).toBeDefined();
+
+      if (showCommand) {
+        const jsonOption = showCommand.options.find((opt) => opt.long === '--json');
+        expect(jsonOption).toBeDefined();
+      }
     });
 
     it('clean command should have force option', () => {