Add an agent-only Spindle runtime that builds and runs natively on macOS.
- keep the full microVM and Nixery engine set Linux-only
- require the worker gateway for agent-only mode
- expose the mode as
spindle run --agent-only - retain shared runtime configuration and database setup
- cover the disabled-gateway failure path
Validated with focused Go tests, a native macOS build, the Linux Nix package build, and spindle-agent interoperability tests.