Make agent-tool subtrees suspendable

Propagate graceful-suspend signals through detached run contexts and let
only opt-in suspendable tools re-attach cancellation, so AsTool sub-agents
can checkpoint and restore across nested trees while leaf tools keep
running detached.

Add focused agent and worker tests for single and multi-level suspend/
restore flows, plus heartbeat lease-loss and nested-restore error paths to
harden functional behavior under failure conditions.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/agent/tool.go

@@ -39,6 +39,17 @@ type (
 		ToolDescriptor
 		Execute(ctx context.Context, arguments string) (ToolResult, error)
 	}
+
+	// SuspendableTool marks tools that can safely receive the run's
+	// graceful-suspend signal and checkpoint their own progress.
+	//
+	// Leaf tools should generally not implement this interface: they are
+	// expected to run on a detached context so in-flight side effects are
+	// not aborted during shutdown.
+	SuspendableTool interface {
+		Tool
+		Suspendable()
+	}
 )
 
 // ResultJSON marshals v to JSON and returns a successful ToolResult.