Add Elixir markdown pipeline with org-citar citation resolution

Introduces scripts/pipeline/, a Mix project that runs as a post-export
transformation pass over content/*.md before Quartz builds the site.

Pipeline (scripts/export.exs phase 3):
- Compiles and loads the Mix project at export time (cached after first run)
- Applies a list of Transform modules sequentially over all .md files
- Only rewrites files that were actually changed

Citations transform (Pipeline.Transforms.Citations):
- Resolves [cite:@key] and bare cite:key syntax produced by ox-hugo/citar
- Resolution chain: Zotero BBT JSON-RPC → BibTeX file → DOI/bare-key fallback
- Zotero probe uses a no-op JSON-RPC call (cayw endpoint blocks indefinitely)
- Zotero resolver fetches PDF attachments via item.attachments, producing
  zotero://open-pdf/... links; falls back to zotero://select/library/items/...
- BibTeX resolver parses .bib files with a simple regex parser (no deps)
- DOI resolver is the always-succeeding last resort

Configuration via env vars:
  BIBTEX_FILE   — path to .bib file for fallback resolution
  ZOTERO_URL    — Zotero base URL (default: http://localhost:23119)
  CITATION_MODE — silent | warn (default) | strict

Adding future transforms requires only implementing Pipeline.Transform
behaviour and appending the module to the transforms list in export.exs.

scripts/pipeline/mix.exs

@@ -0,0 +1,27 @@
+defmodule Pipeline.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :pipeline,
+      version: "0.1.0",
+      elixir: "~> 1.15",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  def application do
+    [
+      extra_applications: [:logger, :inets, :ssl],
+      mod: {Pipeline.Application, []}
+    ]
+  end
+
+  defp deps do
+    [
+      {:req, "~> 0.5"},
+      {:jason, "~> 1.4"}
+    ]
+  end
+end