Configure prettier and make fmt-check cover markdown (closes #69)

Adds .prettierrc and .prettierignore, a single script/prettier entrypoint shared by fmt and fmt-check so the two cannot drift, and prettier 3.9.6 pinned by yarn.lock integrity hash.

Markdown formatting is now gated in the authoritative Docker build via a new mdfmt stage, since the golangci-lint image has no node. REPO_POLICIES.md is ignored so local tooling cannot drift it from upstream.

Removes the || true that made the previous prettier invocation unable to fail.

Makefile

@@ -13,7 +13,7 @@ GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Gitrev=$(GITREV_BUILD)
 GOFLAGS := -ldflags "$(GOLDFLAGS)"
 
-.PHONY: bootstrap setup docker default run ci test check lint fmt fmt-check hooks fixme
+.PHONY: bootstrap setup docker default run ci test check lint fmt fmt-check fmt-check-go fmt-check-md hooks fixme
 
 default: fmt test
 
@@ -44,6 +44,14 @@ check:
 fmt-check:
 	@script/fmt-check
 
+# Halves of fmt-check, for environments that have only one toolchain:
+# the Docker lint stage has Go but no node, the markdown stage the reverse.
+fmt-check-go:
+	@script/fmt-check-go
+
+fmt-check-md:
+	@script/prettier --check
+
 hooks:
 	@script/install-precommit