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

Makefile

@@ -1,13 +1,14 @@
 ROOT_DIR := $(shell pwd)
 PYTHON ?= python3
 
-.PHONY: help install uninstall sync-skill
+.PHONY: help install uninstall sync-skill test
 
 help:
 	@echo "Usage: make [target]"
 	@echo ""
 	@echo "  install     Install mytoolkit in editable mode (current Python) and sync skill"
 	@echo "  sync-skill  Copy the bundled agent skill to ~/.agents/skills/mytoolkit/"
+	@echo "  test        Run the pytest suite"
 	@echo "  uninstall   Uninstall mytoolkit and remove shell completions"
 
 install:
@@ -28,3 +29,11 @@ sync-skill:
 
 uninstall:
 	@mytoolkit uninstall || $(PYTHON) -m pip uninstall mytoolkit -y
+
+test:
+	@cd "$(ROOT_DIR)" && \
+	if command -v uv >/dev/null 2>&1; then \
+		uv run pytest tests/ -q; \
+	else \
+		$(PYTHON) -m pytest tests/ -q; \
+	fi