fix(agent): suppress plan nudge and SkipPlaceholder for system messages

When a subagent completes, processSystemMessage triggers runAgentLoop
with SendResponse: true. This caused three problems:

1. The plan-execution nudge fired, forcing the LLM into unnecessary
   iterations when it was correctly waiting for user input
2. The response was sent without SkipPlaceholder, consuming the
   Telegram "Thinking..." placeholder and corrupting status messages
3. Tool call results from the nudged iterations leaked into the chat

Add SystemMessage flag to processOptions. When set:
- Plan continuation nudge is suppressed in runLLMIteration
- Outbound response uses SkipPlaceholder: true

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/loop.go

@@ -115,6 +115,7 @@ type processOptions struct {
 	NoHistory       bool   // If true, don't load session history (for heartbeat)
 	TaskID          string // Unique task ID for background task status tracking
 	Background      bool   // If true, this is a background task (cron/heartbeat) — enables live task notifications
+	SystemMessage   bool   // If true, this is a system/subagent message — suppress plan nudge, use SkipPlaceholder
 }
 
 const defaultResponse = "I've completed processing but have no response to give. Increase `max_tool_iterations` in config.json."
@@ -816,6 +817,7 @@ func (al *AgentLoop) processSystemMessage(ctx context.Context, msg bus.InboundMe
 		DefaultResponse: "Background task completed.",
 		EnableSummary:   false,
 		SendResponse:    true,
+		SystemMessage:   true,
 	})
 }
 
@@ -1228,6 +1230,7 @@ func (al *AgentLoop) runAgentLoop(ctx context.Context, agent *AgentInstance, opt
 			Channel:         opts.Channel,
 			ChatID:          opts.ChatID,
 			Content:         finalContent,
+			SkipPlaceholder: opts.SystemMessage,
 		})
 	}
 
@@ -2282,7 +2285,7 @@ func (al *AgentLoop) runLLMIteration(
 				curUnchecked = strings.Count(agent.ContextBuilder.ReadMemory(), "- [ ]")
 			}
 			if curUnchecked > 0 && !planMarkNudged &&
-				planSnapshot == "executing" {
+				planSnapshot == "executing" && !opts.SystemMessage {
 				planMarkNudged = true
 				messages = append(messages, providers.Message{
 					Role:    "assistant",