fix(agentic): persist rich model rounds from CLI execution messages

[bobleer]

Summary

• Build model_rounds from execution new_messages in complete_dialog_turn when the host surface (e.g. CLI) does not persist rounds itself.
• Ensures turn files on disk contain rich conversation data (text, tool calls/results, thinking) so other surfaces (e.g. Desktop) can render CLI session history correctly.
• Falls back to a single text-only round when no structured messages are available.

Test plan

• cargo check -p bitfun-core
• Run a CLI agent turn with tool calls and verify the persisted turn file includes model_rounds with text/tool/thinking items
• Open the same session in Desktop and confirm the turn history renders with full detail