Silence glob-loader warning with a draft blog placeholder.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: ANest58

src/data/post/_placeholder.md

@@ -0,0 +1,8 @@
+---
+title: Placeholder
+publishDate: 2020-01-01T00:00:00Z
+draft: true
+excerpt: Kept so the content collection loader has at least one file while the blog is disabled.
+---
+
+This file is intentionally unpublished (`draft: true`). Delete it when you add a real post.