fix: split sync workflow into separate files to fix job skipping issue

The previous combined workflow had flawed conditional logic that caused
sync-keyword and sync-metrics jobs to be skipped. GitHub Actions doesn't
populate github.event.schedule with the cron expression, so the equality
checks never matched.

Replaced with three separate workflow files:
- sync-changes.yml: Runs every 2 minutes
- sync-keyword.yml: Runs every 15 minutes
- sync-metrics.yml: Runs every hour

Each workflow can be manually triggered via workflow_dispatch.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.github/workflows/sync-changes.yml

@@ -0,0 +1,18 @@
+name: Sync NPM Changes Feed
+
+on:
+  schedule:
+    # Run every 2 minutes
+    - cron: '*/2 * * * *'
+  workflow_dispatch:
+
+jobs:
+  sync-changes:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Trigger changes feed sync
+        run: |
+          curl -X POST "${{ secrets.VERCEL_PRODUCTION_URL }}/api/sync/changes" \
+            -H "Authorization: Bearer ${{ secrets.CRON_SECRET }}" \
+            -H "Content-Type: application/json" \
+            -f -s -S -w "\nHTTP Status: %{http_code}\n"