feat: --footer/--header with {page}/{pages} placeholders, center-aligned

md2pdf.py:
- Expose --footer and --header (was dead code before)
- Remove --forms flag (B2 fix — weasyprint 68.1 silently drops <input>)
- Substitution support: {page} → counter(page), {pages} → counter(pages)
- Center-aligned via @bottom-center / @top-center

md2doc.py:
- Add --footer and --header support via Word field codes (PAGE, NUMPAGES)
- Center-aligned paragraph in footer/header
- Same {page}/{pages} substitution API as md2pdf

Both tools:
- GFM task list support: - [ ] → ☐, - [x]/- [X] → ☑ (synced _bulletize)
- Help text explains placeholder syntax with examples

tests/test_bulletize.py

@@ -28,3 +28,19 @@ def test_tilde_fence_also_preserved():
     text = "~~~bash\n-x\n~~~\n"
     result = _bulletize(text)
     assert "-x" in result
+
+
+def test_unchecked_task_list_becomes_empty_checkbox():
+    text = "- [ ] todo item\n"
+    result = _bulletize(text)
+    assert "☐ todo item" in result
+    assert "[ ]" not in result
+
+
+def test_checked_task_list_becomes_checked_checkbox():
+    text = "- [x] done item\n- [X] also done\n"
+    result = _bulletize(text)
+    assert "☑ done item" in result
+    assert "☑ also done" in result
+    assert "[x]" not in result
+    assert "[X]" not in result