blog-publish: static blog post pipeline

Configuration

Set these once (kit.toml, .env or arguments).

Setting	Meaning	Example
`BLOG_DIR`	Repo folder of your static blog	`./my-blog/`
`POSTS_DIR`	Markdown posts, relative to `BLOG_DIR`	`content/posts/`
`BUILD_CMD`	Command that builds the site	`python3 build.py`
`SITE_DIR`	Build output folder	`site/`
`BLOG_URL`	Public base URL	`https://blog.example.com`
`POST_PATH`	URL pattern for a post	`/blog/<slug>/`
`DEPLOY_CMD`	Your deploy command	see Deploy
`SOCIAL_HANDLES`	Accounts used for distribution	per platform

The steps below assume a build script that renders POSTS_DIR/<slug>.md into SITE_DIR. Adjust names to your generator (Hugo, Eleventy, Astro, a custom script).