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

package.json

@@ -17,7 +17,10 @@
     "check": "tsc --noEmit && npx prettier . --check",
     "format": "npx prettier . --write",
     "test": "tsx --test",
-    "profile": "0x -D prof ./quartz/bootstrap-cli.mjs build --concurrency=1"
+    "profile": "0x -D prof ./quartz/bootstrap-cli.mjs build --concurrency=1",
+    "export": "elixir scripts/export.exs",
+    "build:notes": "elixir scripts/export.exs && npx quartz build",
+    "serve:notes": "elixir scripts/export.exs && npx quartz build --serve"
   },
   "engines": {
     "npm": ">=10.9.2",