hive-runtime: TurnStop replaces the subagent daemon's Stopper

TurnStop is the enum over hive-claude's Cancel and the ACP Canceller, with
stop() -> bool (false when no turn was in flight). It signals SIGINT only and
never waits. The subagent daemon tracks a TurnStop per turn, so interrupt has
one stop path; whether the name stays claimed afterwards (claude) or is freed
(ACP) still follows the runtime.

hive-runtime/src/lib.rs

@@ -20,6 +20,7 @@ use std::path::PathBuf;
 mod acp;
 mod claude;
 mod spec;
+mod stop;
 
 pub use acp::{
     AcpError, AcpRuntime, Canceller, Choice, Choices, PermissionAsk, PermissionPolicy,
@@ -33,6 +34,7 @@ pub use spec::{
     ACP_API_KEY_ENV_ENV, ACP_ARGS_ENV, ACP_COMMAND_ENV, ACP_ENV_ENV, AcpCommand, RUNTIME_ENV,
     RuntimeSpec, SpecError,
 };
+pub use stop::TurnStop;
 
 /// The operations every runtime backend offers.
 ///