Add agents.md for AI-assisted development context

Provides MkDocs setup, navigation conventions, blog post structure,
and markdown extension usage for the documentation site.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

agents.md

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+# Documentation
+
+Developer and user documentation site for Freeshard, published at docs.freeshard.net. Built with MkDocs and the Material theme.
+
+## Tech Stack
+
+- **Generator**: MkDocs with Material theme
+- **Plugins**: blog, glightbox (image lightbox)
+- **Markdown extensions**: admonitions, details, syntax highlighting, Mermaid diagrams, emoji, markdown-include
+- **Dependencies**: `requirements.txt` (mkdocs, mkdocs-material, markdown-include, mkdocs-glightbox)
+
+## Commands
+
+```bash
+pip install -r requirements.txt   # Install dependencies
+mkdocs serve                      # Dev server on localhost:8000
+mkdocs build                      # Generate static site to public/
+```
+
+## Structure
+
+```
+docs/
+  overview/              Product overview and concepts
+    concepts/              Single-user isolation, devices, apps
+  developer_docs/        Developer documentation for app creators
+    includes/              Reusable markdown snippets (template vars, portal name info)
+    img/                   Developer docs images
+  user_guides/           End-user guides (password management, smart home)
+  blog/
+    posts/                 Blog posts (date-prefixed directories)
+      YYYY-MM-DD_slug/       Each post: main.md + images
+    .authors.yml           Blog author metadata
+  css/extra.css          Custom styles
+  img/                   Shared images (logo)
+mkdocs.yml               Site config, navigation, theme, plugins
+```
+
+## Conventions
+
+### Navigation
+Navigation is explicitly defined in `mkdocs.yml` under the `nav` key. Four top-level sections: Overview, Developer Docs, Blog, User Guides. Adding a new page requires adding it to `nav`.
+
+### Blog Posts
+Blog posts live in `docs/blog/posts/YYYY-MM-DD_slug/main.md`. Each post directory contains the markdown file and any associated images. Authors are defined in `docs/blog/.authors.yml`.
+
+### Markdown Includes
+Reusable snippets in `docs/developer_docs/includes/` can be included in other docs via the `markdown-include` extension: `{!developer_docs/includes/snippet.md!}`.
+
+### Images
+Store images alongside the content that uses them (in subdirectory `img/` or in blog post directories). Use relative paths. The glightbox plugin automatically adds lightbox behavior to images.
+
+### Mermaid Diagrams
+Supported via `pymdownx.superfences` custom fence. Use ` ```mermaid ` code blocks.
+
+## Deployment
+
+Built and deployed via GitLab Pages. The `site_dir` is `public/` (GitLab Pages convention). Site URL: `https://docs.freeshard.net`.