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.

AGENTS.md

@@ -13,7 +13,8 @@ source for coding standards, formatting, linting, and workflow rules.
 - After each change, run `make fmt`, then `make test`, then `make lint`. Fix any
   failures before committing.
 
-- After each change, commit only the files you've changed. Push after committing.
+- After each change, commit only the files you've changed. Push after
+  committing.
 
 ## Attribution
 
@@ -26,5 +27,5 @@ source for coding standards, formatting, linting, and workflow rules.
 - The proto definition is in `mfer/mf.proto`; generated `.pb.go` files are
   committed (required for `go get` compatibility).
 - The format specification is in `FORMAT.md`.
-- See the TODO section in `README.md` for the 1.0 implementation plan
-  and open design questions.
+- See the TODO section in `README.md` for the 1.0 implementation plan and open
+  design questions.