Add Nix-built Go program and agent rules

OneHouse starts as a program that prints its name, with the rules and
tools for all later work. The program gives the Nix package a binary
that the install check can run, so the build pipeline is tested before
the first feature.

The agent guide and its skills hold the rules for Go code,
documentation, checks, commits, and issues. The scripts in bin/ format
and check the repository with the tools from the pinned flake, and the
commit hook rejects a message that breaks the message rules.