Handle default harness correctly when resume a conversation.

[wjjclaud]

Fix #306

• Resume a conversation should use the same harness stored in its log. Resume doesn't need to explicitly specify the harness via --harness flag.
• Reject only an explicit harness ID is passed on resume.
• In the conversation log, record the actual harness ID instead of empty string "" when starting a conversation with default harness.

Tested

Rerun the sequence in #306 without error.