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.
Adopts golangci-lint v2.12.2 and the canonical .golangci.yml (default: all), and fixes all resulting findings across the tree.
Two intended behavior changes: absent MFFilePath.Mtime is handled explicitly in freshen, list and export rather than dereferenced (main panicked); gpg positional key IDs now follow an explicit -- end-of-options marker.
All twelve reworded user-visible error messages restored to byte-identical parity with main and pinned by tests.
