test: add pytest suite — CLI smoke + config/templates/md_to_pdf units

- tests/conftest.py redirects MYTOOLKIT_HOME to a temp dir before imports
  so the module-level Config singleton never touches the real ~/.mytoolkit
- CLI smoke: every registered command answers --help with exit code 0
- unit tests: Config get/set/migrate, templates_registry override/fallback,
  md_to_pdf pure helpers (pandoc-dependent cases skipif-guarded)
- pyproject: dev dependency group (pytest) + pytest config; Makefile: make test

tests/test_templates_registry.py

@@ -0,0 +1,44 @@
+"""Tests for mytoolkit.templates_registry (external root override + fallback)."""
+
+import click
+import pytest
+
+from mytoolkit import templates_registry as tr
+
+
+@pytest.fixture(autouse=True)
+def clean_registry(tmp_path, monkeypatch):
+    """Point the registry config file at a temp location for every test."""
+    monkeypatch.setattr(tr, "CONFIG_PATH", tmp_path / "templates.json")
+    yield
+
+
+def test_default_root_is_bundled_package():
+    root = tr.get_root()
+    assert (root / "md-to-pdf").is_dir()
+    assert (root / "md-to-docx").is_dir()
+
+
+def test_set_root_overrides(tmp_path):
+    ext = tmp_path / "ext-templates"
+    (ext / "md-to-pdf").mkdir(parents=True)
+    (ext / "md-to-docx").mkdir()
+    assert tr.set_root(ext) == ext.resolve()
+    assert tr.get_root() == ext.resolve()
+    assert tr.get_md_to_pdf_root() == ext.resolve() / "md-to-pdf"
+    assert tr.get_md_to_docx_root() == ext.resolve() / "md-to-docx"
+
+
+def test_invalid_registered_dir_falls_back_to_bundled(tmp_path):
+    tr.set_root(tmp_path / "does-not-exist")
+    root = tr.get_root()
+    assert (root / "md-to-pdf").is_dir()
+    assert root != tmp_path / "does-not-exist"
+
+
+def test_missing_subdir_raises_usage_error(tmp_path):
+    ext = tmp_path / "ext-templates"
+    ext.mkdir()  # no md-to-pdf inside
+    tr.set_root(ext)
+    with pytest.raises(click.UsageError):
+        tr.get_md_to_pdf_root()