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/agent_tool.go

@@ -41,6 +41,8 @@ type (
 
 var (
 	agentToolParamsSchema = mustJSONSchemaFor[agentToolParams]()
+
+	_ SuspendableTool = (*agentTool)(nil)
 )
 
 func agentToolDepth(ctx context.Context) int {
@@ -62,6 +64,8 @@ func newAgentTool(agent *Agent, name, description string) *agentTool {
 
 func (t *agentTool) Name() string { return t.toolName }
 
+func (t *agentTool) Suspendable() {}
+
 func (t *agentTool) Definition() llm.Tool {
 	return llm.Tool{
 		Name:        t.toolName,