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

opencode.json

@@ -0,0 +1,10 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "mcp": {
+    "nixos": {
+      "type": "local",
+      "command": ["mcp-nixos"],
+      "enabled": true
+    }
+  }
+}