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

flake.nix

@@ -0,0 +1,30 @@
+{
+  description = "Quartz org-roam dev shell";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = [
+            pkgs.nodejs_22
+            pkgs.elixir
+            ((pkgs.emacsPackagesFor pkgs.emacs-nox).emacsWithPackages (epkgs: [ epkgs.ox-hugo ]))
+            pkgs.mcp-nixos
+          ];
+
+          shellHook = ''
+            echo "Node $(node --version) / npm $(npm --version)"
+            elixir --version 2>/dev/null | head -1 || true
+            echo "Emacs $(emacs --version | head -1)"
+          '';
+        };
+      });
+}