fix(attach): report resolved agent name in empty-transcript warning

The empty-transcript warning printed the raw --agent flag value, but the transcript is parsed from the agent resolved in resolveAgentAndTranscript, which can come from existing session state or auto-detection. When the agent was auto-detected or read from state, the warning pointed the user at the wrong --agent value. Report ag.Name() (the resolved agent's flag-form name) instead.