Add conversation titles and rename support

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Summary

Conversation persistence currently stores only the message transcript in ~/.agent/history.yaml, so saved chats have no human-friendly titles or metadata.

Proposed change

• Generate a title for new conversations from the first exchange.
• Allow renaming the active conversation from the TUI.
• Persist the title alongside the transcript.

Acceptance criteria

• Saved chats display a title instead of appearing as untitled transcripts.
• Users can rename the current conversation from the UI.
• Existing transcripts continue to load even if they have no title yet.