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/metro/metro-madrid.org

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+:PROPERTIES:
+:ID:       metro-madrid
+:END:
+#+title: Metro de Madrid
+
+The Madrid Metro is the main rapid transit network in the city, opened in 1919.
+It is the second oldest metro in the Iberian Peninsula after Barcelona.
+
+* Key Lines
+| Line | Name            | Colour | Terminals                    |
+|------+-----------------+--------+------------------------------|
+| L1   | Pinar de Chamartín–Valdecarros | Blue   | Pinar de Chamartín / Valdecarros |
+| L6   | Circular        | Grey   | Circular (loop)              |
+| L10  | —               | Dark blue | Hospital Infanta Sofía / Tres Olivos |
+
+* See also
+- [[id:madrid-transport][Madrid Public Transport]]
+- [[id:sol-interchange][Sol interchange]]

notes/metro/sol-interchange.org

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+:PROPERTIES:
+:ID:       sol-interchange
+:END:
+#+title: Sol (interchange)
+
+Sol is the busiest interchange station in the Madrid Metro, sitting beneath
+Puerta del Sol in the city centre.
+
+Lines serving Sol: [[id:metro-madrid][L1]], L2, L3.
+
+It also connects to the Cercanías hub underneath, making it the de-facto
+zero point of Madrid's public transport.