Add a `pi` module that turns on the home-manager `programs.pi-coding-agent`
integration for the primary user and freezes a single settings.json: the
default provider set to Anthropic, the default model to Opus (claude-opus-4-8,
Pi's own catalogue id), and analytics disabled. Everything else in
~/.pi/agent — the credential and all self-modification state — is left
unmanaged, so login survives rebuilds and backing Pi out is a one-line enable
flip. Enabled on neogaia alone.
