Clean output directory before generating themes

Prevents stale files from accumulating when themes are renamed or removed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

generate.ts

@@ -1,4 +1,4 @@
-import { readdir, writeFile, mkdir } from 'node:fs/promises';
+import { readdir, writeFile, mkdir, rm } from 'node:fs/promises';
 import { join, resolve } from 'node:path';
 import { loadTheme } from './src/palette.js';
 import { buildVariables } from './src/themeBuilder.js';
@@ -15,6 +15,7 @@ async function findDefinitionFiles(dir: string): Promise<string[]> {
 }
 
 async function main(): Promise<void> {
+  await rm(OUTPUT_DIR, { recursive: true, force: true });
   await mkdir(OUTPUT_DIR, { recursive: true });
 
   const definitionsDir = join(DOKI_REPO, 'definitions');