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.

.prettierrc

@@ -0,0 +1,4 @@
+{
+    "tabWidth": 4,
+    "proseWrap": "always"
+}