build: add the Makefile command surface (D-27)

Every repo gets one at the root: help, plus test and lint where those exist.
The point is that a target name means the same thing in every repo, so an
agent or a person can act without reading the repo first.

Paths resolve here rather than in callers (D-10). python3 on this host is 3.8
and cannot parse these sources, and a bare pytest or ruff resolves only in a
login shell — so both are named explicitly through the venv, and a missing
venv fails with the command to fix it rather than a bare no-such-file.

Co-Authored-By: Claude <noreply@anthropic.com>

Makefile

@@ -0,0 +1,33 @@
+# tatlock-ui — the repo's command surface (D-27).
+#
+# Flutter rather than Python, so there is no venv and no PYTHON here. The
+# reason the targets still exist under these names is the point of D-27: an
+# agent or a person can run `make test` in any repo in this workspace without
+# first working out which stack it is.
+
+.DEFAULT_GOAL := help
+
+.PHONY: help
+help: ## Show this help
+	@grep -hE '^[a-z][a-z0-9_-]*:.*?## ' $(MAKEFILE_LIST) \
+	  | awk 'BEGIN{FS=":.*?## "}{printf "  \033[36m%-14s\033[0m %s\n", $$1, $$2}'
+
+.PHONY: setup
+setup: ## Fetch package dependencies
+	flutter pub get
+
+.PHONY: test
+test: ## Run the widget and unit tests
+	flutter test
+
+.PHONY: lint
+lint: ## Static analysis (analysis_options.yaml at the repo root)
+	flutter analyze
+
+.PHONY: build
+build: ## Release build for the web target
+	flutter build web --release
+
+.PHONY: clean
+clean: ## Remove build artefacts and the pub cache for this project
+	flutter clean