Prove the crew sandbox boundary with a real sandboxed shell: own server reachable, outside localhost and every denyRead path refused #1234

#1107 / #1108 keep the boundary (no unix sockets let through) and test the settings that enforce it, but test/crew-sandbox.test.ts cannot start a real sandboxed shell from inside a crew sandbox (sandboxes do not nest). Add a test that runs where a sandbox can be created (the forge check, outside a crew run): spawn a shell under the generated settings, assert it reaches a loopback server it started, is refused localhost ports outside the sandbox, and cannot read any denyRead path. Skip with a named reason when nested.