Add Copilot instructions

.github/copilot-instructions.md

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+# Copilot Instructions — Blog
+
+Personal weblog built with Jekyll 4 and the Minima theme, deployed via Netlify.
+
+## Stack
+
+- **Framework:** Jekyll ~4.0 (Ruby)
+- **Theme:** Minima ~2.0
+- **Markdown:** kramdown with GFM input
+- **Styles:** SASS (compressed), source in `_sass/`
+- **Deploy:** Netlify (`netlify.toml`)
+
+## Commands
+
+| Task | Command |
+|------|---------|
+| Install deps | `bundle install` |
+| Serve locally | `bundle exec jekyll serve` |
+| Build | `bundle exec jekyll build` |
+
+## Project Structure
+
+```
+_config.yml          # Site config, plugin list, permalink pattern
+_layouts/            # Page layouts
+_includes/           # Reusable partials
+_sass/               # SASS stylesheets
+_data/               # Data files
+assets/              # Static assets (images, fonts)
+collections/         # Content collections (software_development, miscellaneous, cascadia)
+css/                 # Compiled CSS output
+js/                  # JavaScript
+```
+
+## Content Conventions
+
+### Posts & Collections
+
+- Posts live in `collections/` under their collection name (e.g., `collections/_software_development/`)
+- Collections defined in `_config.yml`: `software_development`, `miscellaneous`, `cascadia`
+- Permalink pattern: `/weblog/:categories/:year/:month/:day/:title.html`
+- Use `<!--description-->` as the excerpt separator
+
+### Front Matter
+
+Every post/page requires Jekyll front matter. Match existing patterns — typically `layout`, `title`, `date`, `tags`, and `categories`.
+
+### Archives
+
+Jekyll Archives plugin generates year, month, day, category, and tag archive pages automatically. Layouts: `year-archive`, `month-archive`, `day-archive`, `category-archive`, `tag-archive`.
+
+## Plugins
+
+- `jekyll-feed` — Atom feed generation
+- `jekyll-archives` — Date/category/tag archives
+- `jekyll_picture_tag` — Responsive images
+- `jekyll-autoprefixer` — CSS vendor prefixes
+- `jekyll-timeago` — Relative time formatting
+
+## Style Guidelines
+
+- Match existing SASS patterns in `_sass/`
+- Do not add new CSS frameworks — the site uses Minima + custom SASS
+- Preserve the existing permalink structure
+- Keep page templates consistent with existing layouts