Add org-roam workflow: ox-hugo export pipeline and example notes

- Add flake.nix dev shell with Node.js 22, Elixir, and Emacs+ox-hugo
- Add scripts/export.exs: exports org-roam notes to content/ via ox-hugo,
  mirroring subdirectory structure and generating a fallback index.md
- Add npm scripts: export, build:notes, serve:notes
- Configure FrontMatter plugin for TOML (ox-hugo default output)
- Replace ObsidianFlavoredMarkdown with OxHugoFlavouredMarkdown
- Add example notes: Madrid public transport (metro, bus, roads)
- Update README and AGENTS.md with org-roam workflow instructions

AGENTS.md

@@ -17,6 +17,16 @@ Built with TypeScript, Preact, and unified/remark/rehype for markdown processing
 | Build Tool    | esbuild                                   |
 | Styling       | SCSS via esbuild-sass-plugin              |
 
+## Environment
+
+This is a Nix project. Use the provided `flake.nix` to enter a dev shell with Node.js 22 and npm:
+
+```bash
+nix develop
+```
+
+All `npm` commands below must be run inside the dev shell.
+
 ## Build, Lint, and Test Commands
 
 ```bash
@@ -234,6 +244,36 @@ git branch -d feature/my-feature
 
 **Merge direction:** `upstream → main → org-roam → feature/*`
 
+## Org-Roam Workflow
+
+Notes live in a **separate directory** outside this repo. The export script
+converts them to Markdown via ox-hugo, then Quartz builds the site.
+
+### Tooling
+
+The dev shell (`nix develop`) provides:
+
+- `nodejs_22` — Quartz build
+- `elixir` — runs the export script
+- `emacs` + `ox-hugo` — performs the org → markdown conversion
+
+### Export and build
+
+```bash
+# Export only (wipes content/, exports all .org files)
+NOTES_DIR=/path/to/notes npm run export
+
+# Export then build the site
+NOTES_DIR=/path/to/notes npm run build:notes
+
+# Positional arg also works
+elixir scripts/export.exs /path/to/notes
+```
+
+The export script (`scripts/export.exs`) runs Emacs in batch mode, calling
+`org-hugo-export-to-md` per file (per-file mode, not per-subtree). It uses
+YAML frontmatter (ox-hugo default). `content/` is wiped before each export.
+
 ## Important Notes
 
 - **Client-side scripts**: Use `.inline.ts` suffix, bundled via esbuild