Add a Nix flake that builds Pi from the checkout on aarch64/x86_64 Linux
and macOS. The build is offline: provider model data is hydrated from a
pi.dev model catalog revision pinned in nix/model-catalog.json, and the
runtime dependency tree comes from packages/coding-agent/install-lock with
internal packages built from source.
The Nix workflow keeps the catalog pin current on main, builds release
tags on all systems, and fast-forwards the stable branch to each release
that builds. The release script refreshes the pin before tagging.
