For agents, on top of README.md, which they read first.

• Tool paths come from .buckconfig.local [nix], which the devshell writes. Use read_root_config here: a cell's own read_config doesn't see the root's local config.
• -Copt-level must stay explicit: the prelude derives build scripts' OPT_LEVEL from it, and cc-rs fails without that.
• rust.rustc_identity must stay a hard failure when unset. A fallback placeholder would put every compiler under one cache key again and bring back E0514 from the shared cache (hit 2026-09-30).