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

scripts/export.exs

@@ -0,0 +1,136 @@
+#!/usr/bin/env elixir
+# Export org-roam notes (per-file) to content/ via ox-hugo.
+#
+# Usage:
+#   NOTES_DIR=~/notes elixir scripts/export.exs
+#   elixir scripts/export.exs /path/to/notes
+#
+# The positional argument takes precedence over the NOTES_DIR env var.
+
+notes_dir =
+  case System.argv() do
+    [dir | _] -> dir
+    [] ->
+      System.get_env("NOTES_DIR") ||
+        (IO.puts(:stderr, "Usage: NOTES_DIR=/path/to/notes elixir scripts/export.exs"); System.halt(1))
+  end
+
+notes_dir = Path.expand(notes_dir)
+repo_root = __DIR__ |> Path.join("..") |> Path.expand()
+content_dir = Path.join(repo_root, "content")
+
+unless File.dir?(notes_dir) do
+  IO.puts(:stderr, "Error: notes directory does not exist: #{notes_dir}")
+  System.halt(1)
+end
+
+# Wipe content/, preserving .gitkeep
+IO.puts("==> Wiping #{content_dir}")
+
+content_dir
+|> File.ls!()
+|> Enum.reject(&(&1 == ".gitkeep"))
+|> Enum.each(fn entry ->
+  Path.join(content_dir, entry) |> File.rm_rf!()
+end)
+
+# Collect all .org files
+IO.puts("==> Exporting org files from #{notes_dir}")
+
+org_files =
+  Path.join(notes_dir, "**/*.org")
+  |> Path.wildcard()
+
+if org_files == [] do
+  IO.puts("No .org files found in #{notes_dir}")
+  System.halt(0)
+end
+
+# Export each file via emacs --batch
+results =
+  Enum.map(org_files, fn orgfile ->
+    IO.puts("  exporting: #{orgfile}")
+
+    # Mirror the notes subdirectory structure under content/
+    section =
+      orgfile
+      |> Path.dirname()
+      |> Path.relative_to(notes_dir)
+
+    {output, exit_code} =
+      System.cmd(
+        "emacs",
+        [
+          "--batch",
+          "--eval", "(require 'ox-hugo)",
+          "--eval", ~s[(setq org-hugo-base-dir "#{repo_root}")],
+          "--eval", ~s[(setq org-hugo-default-section-directory "#{section}")],
+          "--visit", orgfile,
+          "--funcall", "org-hugo-export-to-md"
+        ],
+        stderr_to_stdout: true
+      )
+
+    # Filter noisy emacs startup lines, same as the shell script
+    filtered =
+      output
+      |> String.split("\n")
+      |> Enum.reject(&String.match?(&1, ~r/^Loading|^ad-handle|^For information/))
+      |> Enum.join("\n")
+
+    if filtered != "", do: IO.puts(filtered)
+
+    {orgfile, exit_code}
+  end)
+
+failures = Enum.filter(results, fn {_, code} -> code != 0 end)
+
+if failures != [] do
+  IO.puts(:stderr, "\nFailed to export #{length(failures)} file(s):")
+  Enum.each(failures, fn {f, code} -> IO.puts(:stderr, "  [exit #{code}] #{f}") end)
+  System.halt(1)
+end
+
+md_count =
+  Path.join(content_dir, "**/*.md")
+  |> Path.wildcard()
+  |> length()
+
+# Generate a default index.md if none was exported
+index_path = Path.join(content_dir, "index.md")
+
+unless File.exists?(index_path) do
+  IO.puts("==> Generating default index.md")
+
+  pages =
+    Path.join(content_dir, "**/*.md")
+    |> Path.wildcard()
+    |> Enum.map(fn path ->
+      slug = Path.relative_to(path, content_dir) |> Path.rootname()
+
+      title =
+        path
+        |> File.read!()
+        |> then(fn content ->
+          case Regex.run(~r/^title\s*=\s*"(.+)"/m, content) do
+            [_, t] -> t
+            _ -> slug
+          end
+        end)
+
+      {slug, title}
+    end)
+    |> Enum.sort_by(fn {_, title} -> title end)
+    |> Enum.map(fn {slug, title} -> "- [#{title}](#{slug})" end)
+    |> Enum.join("\n")
+
+  File.write!(index_path, """
+  ---
+  title: Index
+  ---
+
+  #{pages}
+  """)
+end
+
+IO.puts("==> Done. #{md_count} markdown files in #{content_dir}")