sync: multi-branch scanning, discovery strategies, UTF-8 fix, curation improvements

- Add multi-branch scanning to discover skills on non-default branches
- Add popular repos and commits search discovery strategies
- Fix UTF-8 encoding sanitization in skill parser
- Add repo_created_at for threshold-based duplicate tie-breaking
- Add indexer tests and vitest config

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

services/indexer/vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+  },
+});