fix(tools): block message tool self-send to prevent duplicate media delivery

When a team member creates an image and completes a task, the announce
flow already carries media to the leader agent, which dispatches it to
the channel via ForwardMedia. If the leader also calls the message tool
to send the same image to its own chat, the image appears twice.

Block message tool calls where channel+target match the agent's own
context, returning an error that guides the LLM to include content in
its normal response instead.

internal/tools/message.go

@@ -95,6 +95,15 @@ func (t *MessageTool) Execute(ctx context.Context, args map[string]any) *Result
 		return ErrorResult("target chat ID is required (no current chat in context)")
 	}
 
+	// Block self-send: agent should not use message tool to send to its own
+	// channel/chat — the response is already dispatched via normal outbound flow.
+	// This prevents duplicate media delivery when announce flow already carries media.
+	ctxChannel := ToolChannelFromCtx(ctx)
+	ctxChatID := ToolChatIDFromCtx(ctx)
+	if ctxChannel != "" && ctxChatID != "" && channel == ctxChannel && target == ctxChatID {
+		return ErrorResult("You are already responding to this chat. Your response (including any MEDIA: references) will be delivered automatically. Do not use the message tool to send to your own chat — just include the content in your response text.")
+	}
+
 	// Tenant isolation: validate channel belongs to current tenant.
 	if err := t.validateChannelTenant(ctx, channel, target); err != nil {
 		return err