feat(agent): add TargetAgentID to SubTurnConfig for cross-agent delegation

When TargetAgentID is set, spawnSubTurn resolves the target AgentInstance
from the registry and uses it as the base for the child turn. This gives
the child turn the target's workspace, model, tools, and system prompt
instead of inheriting from the caller.

Model validation is relaxed: empty Model is accepted when TargetAgentID
provides the model implicitly via the resolved agent instance.

Ref: #2148

pkg/tools/subagent.go

@@ -30,6 +30,7 @@ type SubTurnConfig struct {
 	ActualSystemPrompt string
 	InitialMessages    []providers.Message
 	InitialTokenBudget *atomic.Int64 // Shared token budget for team members; nil if no budget
+	TargetAgentID      string        // If set, run as this agent (its workspace, model, tools)
 }
 
 type SubagentTask struct {