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/roads/m30.org

@@ -0,0 +1,19 @@
+:PROPERTIES:
+:ID:       m30
+:END:
+#+title: M-30
+
+The M-30 is Madrid's innermost ring road, circling the city centre at a
+radius of roughly 3–5 km from Puerta del Sol.
+
+It runs mostly underground through the Madrid Río tunnel section along the
+Manzanares river, built during the 2004–2007 renovation that reclaimed the
+riverbank as a public park.
+
+* Key junctions
+- Nudo Norte — connects to A-1 (Burgos) and A-6 (La Coruña)
+- Nudo Sur — connects to A-4 (Cádiz) and A-42 (Toledo)
+
+* See also
+- [[id:crtm][CRTM]]
+- [[id:madrid-transport][Madrid Public Transport]]