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

• pgrx_embed.rs is generated; do not edit it. tools/cargo-gen writes it for every crate whose first_party_library has pgrx_extension = True, and //tools/cargo-gen:drift fails when the committed copy differs.
• It is not in the buck build of the extension beyond the srcs glob; the install SQL comes from tools/pgrx-schema. Do not wire it into :ext.