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.

package.json

@@ -0,0 +1,10 @@
+{
+    "name": "mfer",
+    "version": "0.1.0",
+    "private": true,
+    "description": "Development tooling for the mfer repository: prettier, used by script/fmt and script/fmt-check to format and verify Markdown and JSON.",
+    "license": "WTFPL",
+    "devDependencies": {
+        "prettier": "3.9.6"
+    }
+}