Add supervised runtime sessions with CLI and MCP control

Once now starts long-lived target runs under a supervised session: once runtime start returns a session id, and once runtime status, once runtime logs, and once runtime stop let you inspect or stop the run later. Session stdout, stderr, and status are persisted under <workspace>/.once/runtime/<session>/. For agents, the MCP server exposes the same session model through opt-in tools (once_start_target, once_runtime_status, once_runtime_logs, once_stop_runtime) when started with once mcp --allow-run. The change also keeps Apple-specific runtime conventions in Starlark-declared actions rather than the Rust core, so the core remains runtime-agnostic.