- Build section now recommends the Nix devshell rather than requiring
it; contributors can use their own toolchain/build flow.
- Move the directory-by-directory architecture map out of AGENTS.md
into a new top-level ARCHITECTURE.md, with a pointer left behind.
- Add a short note asking contributors to keep AGENTS.md/README/
ARCHITECTURE.md files current when they change what those docs cover.
- ledger/helpers/AGENTS.md also needs the amendment-gating shape,
per-amendment flag naming, and UNREACHABLE/LCOV_EXCL pairing notes
(previously only in transactors/AGENTS.md) — the vault/lending
helper files these were evidenced on live in ledger/helpers/, not
tx/transactors/.
