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

notes/madrid-transport.org

@@ -0,0 +1,17 @@
+:PROPERTIES:
+:ID:       madrid-transport
+:END:
+#+title: Madrid Public Transport
+
+Madrid has one of the most extensive public transport networks in Europe,
+operated primarily by [[id:crtm][Consorcio Regional de Transportes de Madrid]] (CRTM).
+
+* Modes
+- [[id:metro-madrid][Metro de Madrid]] — 13 lines, ~300 km of track
+- [[id:emt-madrid][EMT Bus]] — urban buses within the city
+- Cercanías — suburban rail run by Renfe
+- Interurbano — regional buses to the wider Community of Madrid
+
+* Ticketing
+A single [[https://www.crtm.es][tarjeta transporte]] (transport card) works across all modes.
+The Multi card covers zones A–C and is topped up at any metro station.