Convert public directory from submodule to normal directory

- Remove public from git submodule tracking
- Add all public directory contents as regular files
- Fixes GitHub Actions error: "No url found for submodule path 'public' in .gitmodules"

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

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

public/book/1/ZGlkOnBsYzo0aHFqZm43bTZuNWhubzNkb2FtdWhnZWY/book.json

@@ -0,0 +1,6 @@
+{
+	"root": "./",
+	"plugins": ["-sharing", "codeblock-filename","mermaid-gb3","diff"],
+	"title": "hello world! bluesky",
+	"description": "This is a bluesky ai-card example, and first rust"
+}