feat: enable authoring posts via GitHub Issues

[atenni]

Problem

I want to be able to author posts from GitHub Issues

Solution

Create a GitHub Action that is:

• Automatically creates a new blog post file in the repository when a GitHub Issue is closed by @atenni and labeled with blog:post, blog:til, or blog:quote.
• Generates a filename and slug from the issue title, ensuring it is lowercase, hyphenated, and sanitised.
• Determines the correct post directory based on the label.
• Writes the entire issue body to the new post file, preserving formatting and YAML front matter.
• Commits and pushes the new post to the repository using the GitHub Actions bot.
• Adds a comment to the original issue with a success message and the post filename.
• Triggers the existing deploy workflow by pushing to the main branch, which will publish the new post to the static site.