feat: accept repository_dispatch so the blog repo can trigger a rebuild

Adds a content-updated dispatch trigger, so publishing a post prerenders it
in about a minute instead of waiting for the daily cron.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: jebin2

.github/workflows/sitemap.yml

@@ -7,6 +7,10 @@ on:
   schedule:
     - cron: '0 3 * * *'
   workflow_dispatch:
+  # Sent by the blog repo right after it publishes, so a new post is
+  # prerendered in about a minute instead of waiting for the daily run.
+  repository_dispatch:
+    types: [content-updated]
   push:
     branches: [main]
     paths: