feat: minor improvements to Makefile

- Make it self-documenting (`make` on its own shows targets)
- Add `make clean` to clean up shrapnel

Makefile

@@ -1,9 +1,24 @@
+# Default target.
+all : commands
+
+## commands : show all commands.
+commands :
+	@grep -h -E '^##' ${MAKEFILE_LIST} | sed -e 's/## //g' | column -t -s ':'
+
+## install: install minimal required packages into current environment.
 install:
 	uv pip install marimo jinja2 markdown
 
+## build: build entire site.
 build:
 	rm -rf _site
 	uv run scripts/build.py
 
+## serve: run local web server without rebuilding.
 serve:
-	uv run python -m http.server --directory _site
+	uv run python -m http.server --directory _site
+
+## clean: clean up stray files.
+clean:
+	@find . -name '*~' -exec rm {} +
+	@find . -name '.DS_Store' -exec rm {} +