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/lib/pipeline.ex

@@ -0,0 +1,83 @@
+defmodule Pipeline do
+  @moduledoc """
+  Post-export markdown transformation pipeline.
+
+  Applies a list of transform modules sequentially over every .md file
+  in a content directory. Each transform module must implement:
+
+      apply(content :: String.t(), opts :: map()) :: String.t()
+
+  Transforms are applied in the order given. A file is rewritten only
+  when at least one transform mutates its content (checked via equality).
+
+  ## Usage
+
+      opts = %{
+        zotero_url: "http://localhost:23119",
+        bibtex_file: System.get_env("BIBTEX_FILE"),
+        citation_mode: :warn   # :silent | :warn | :strict
+      }
+
+      Pipeline.run(content_dir, [Pipeline.Transforms.Citations], opts)
+  """
+
+  require Logger
+
+  @type transform :: module()
+  @type opts :: map()
+
+  @doc """
+  Run all transforms over every .md file under `content_dir`.
+  Returns `{:ok, stats}` where stats maps each transform to a count of files it changed.
+  """
+  @spec run(String.t(), [transform()], opts()) :: {:ok, map()}
+  def run(content_dir, transforms, opts \\ %{}) do
+    md_files =
+      content_dir
+      |> Path.join("**/*.md")
+      |> Path.wildcard()
+
+    if md_files == [] do
+      Logger.warning("Pipeline: no .md files found in #{content_dir}")
+      {:ok, %{}}
+    else
+      Logger.info("Pipeline: processing #{length(md_files)} markdown files with #{length(transforms)} transform(s)")
+
+      # Initialise transforms (allows them to perform setup such as loading a .bib file).
+      # Each transform module must implement the Pipeline.Transform behaviour.
+      initialized =
+        Enum.map(transforms, fn mod ->
+          state = mod.init(opts)
+          {mod, state}
+        end)
+
+      stats =
+        Enum.reduce(md_files, %{}, fn path, acc ->
+          original = File.read!(path)
+
+          {transformed, file_stats} =
+            Enum.reduce(initialized, {original, %{}}, fn {mod, state}, {content, fstats} ->
+              result = mod.apply(content, state, opts)
+              changed = result != content
+              {result, Map.update(fstats, mod, (if changed, do: 1, else: 0), &(&1 + (if changed, do: 1, else: 0)))}
+            end)
+
+          if transformed != original do
+            File.write!(path, transformed)
+            Logger.debug("Pipeline: updated #{Path.relative_to_cwd(path)}")
+          end
+
+          Map.merge(acc, file_stats, fn _k, a, b -> a + b end)
+        end)
+
+      Enum.each(initialized, fn {mod, state} ->
+        # teardown/1 is optional in the behaviour
+        if function_exported?(mod, :teardown, 1) do
+          mod.teardown(state)
+        end
+      end)
+
+      {:ok, stats}
+    end
+  end
+end