Add dev Makefile; format all markdown with prettier

Adds a minimal Makefile wrapping the toolchain the way sneak's other
repos expose it:

- fmt        gofmt -w plus prettier (4-space tabs, proseWrap: always)
- fmt-check  fail if any Go or Markdown file is unformatted
- lint       golangci-lint run ./...
- test       go test ./...
- check      fmt-check + lint + test (the local pre-commit gate)

Running make fmt normalizes the four existing Markdown docs to the
shared prettier style (80-column proseWrap: always, aligned tables) —
a one-time reflow with no content change. The repo remains exempt from
the rest of the policy scaffold (no Dockerfile, CI, or REPO_POLICIES).

Makefile

@@ -0,0 +1,35 @@
+# Development convenience targets. This repo is exempt from the standard
+# policy scaffold (no Dockerfile, CI, or REPO_POLICIES.md); this Makefile
+# is only a thin wrapper around the Go toolchain, golangci-lint, and
+# prettier so `make fmt` / `make check` behave the same as in sneak's
+# other repos.
+
+GO_PKGS  := ./...
+MD_FILES := $(shell git ls-files '*.md')
+PRETTIER := prettier --tab-width 4 --prose-wrap always
+
+.PHONY: check fmt fmt-check lint test
+
+# Format, lint, and test — the full local pre-commit gate.
+check: fmt-check lint test
+
+# Format Go and Markdown in place.
+fmt:
+	gofmt -w .
+	$(PRETTIER) --write $(MD_FILES)
+
+# Fail if any Go or Markdown file is not formatted.
+fmt-check:
+	@unformatted="$$(gofmt -l .)"; \
+	if [ -n "$$unformatted" ]; then \
+		echo "gofmt needed on:"; echo "$$unformatted"; exit 1; \
+	fi
+	$(PRETTIER) --check $(MD_FILES)
+
+# Run the house linter (config in .golangci.yml).
+lint:
+	golangci-lint run $(GO_PKGS)
+
+# Run the test suite.
+test:
+	go test $(GO_PKGS)