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/bus/emt-madrid.org

@@ -0,0 +1,16 @@
+:PROPERTIES:
+:ID:       emt-madrid
+:END:
+#+title: EMT Madrid (urban bus)
+
+Empresa Municipal de Transportes (EMT) operates the urban bus network
+within the municipality of Madrid — around 200 lines.
+
+* Notable lines
+- *Line 27* — connects Embajadores with Barrio de la Concepción, one of the
+  oldest routes in the network.
+- *Line 34* — Argüelles to Carabanchel, crossing the city centre via Gran Vía.
+- *Búho (owl) lines* — night buses running from Cibeles from midnight to 6 am.
+
+* See also
+- [[id:madrid-transport][Madrid Public Transport]]