Listener behaviour for evoq.

Listeners receive integration facts from external transports (pg or mesh) and dispatch commands to the local aggregate. They are the entry point for cross-domain integration.

Listeners get their own slice directory in the target CMD app, like process managers. This gives filesystem-level discoverability of external integration points.

Naming convention: on_{fact}_from_{transport}_{command}

Required Callbacks

- source_fact() -> binary() - transport() -> pg | mesh - handle_fact(FactType, Payload, Metadata) -> ok | skip | {error, Reason}