fix(agent): suppress duplicate final reply after named-agent message tool send

[Alix-007]

Summary

• avoid duplicate outbound replies when a routed named agent already sent a user-visible message via the message tool in the same round
• route final outbound dedupe through the routed session/agent instead of always depending on the default-agent lane
• add regression tests for named-agent message-tool dedupe in both helper and Run flow

Fixes #1909

Verification

• go test ./pkg/agent -run 'TestPublishResponseIfNeeded_SkipsDuplicateForNamedAgentMessageTool|TestRun_NamedAgentMessageToolSuppressesFinalOutbound|TestProcessMessage_PublishesToolFeedbackWhenEnabled' -count=1
  • ok github.com/sipeed/picoclaw/pkg/agent 0.452s