Shim make dev and make build, format backend/ markdown (closes #28)

make dev ran yarn dev inline and there was no make build. Both are now
shims, over script/dev and the new script/build. .prettierignore stops
leaving out backend/, so backend/README.md is formatted and checked;
backend/.golangci.yml is left out by name instead: it is the org
standard file whose sha256 backend/script/lint checks, and prettier
would reindent it. .claude/ stays in .prettierignore, as git does not
ignore it. script/install-precommit and the date on bootstrap's pins go
back to the org model; bootstrap, fmt and fmt-check keep what the Go
backend and eslint need, each with a comment saying so.

Model: opus-5-5

Makefile

@@ -1,5 +1,5 @@
-.PHONY: bootstrap setup dev test lint fmt fmt-check check frontend-check \
-	frontend-viewport-test docker hooks
+.PHONY: bootstrap setup dev build test lint fmt fmt-check check \
+	frontend-check frontend-viewport-test docker hooks
 
 # Standard targets are thin shims; the implementations live in script/
 # per the scripts-to-rule-them-all pattern (see the Entrypoints section
@@ -13,7 +13,11 @@ setup:
 	@script/setup
 
 dev:
-	yarn dev
+	@script/dev
+
+# The frontend only; backend/Makefile's build target builds the Go server.
+build:
+	@script/build
 
 test:
 	@script/test