docs: Add blog post link and convert logo to PNG format

- Add Medium blog post link about slash commands to README files
- Convert claude-howto-logo.svg to PNG format (1024x256)
- Replace old claude-ai-icon.webp with new logo
- Update .gitignore to exclude blog-posts directory

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

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

.gitignore

@@ -44,4 +44,5 @@ yarn-error.log*
 *.tmp
 *.temp
 # Keep .claude/ clean for hand-on section
-.claude/
+.claude/
+blog-posts/