Session manager changes:
- Add ThinkingLevelChangeEntry and ModelChangeEntry types
- Add saveThinkingLevelChange() and saveModelChange() methods
- Update loadThinkingLevel() to also check for thinking_level_change events
(not just session headers)
TUI changes:
- Pass SessionManager to TuiRenderer constructor
- Call saveThinkingLevelChange() when user changes thinking level via /thinking
- Store session manager as instance variable for future use
This ensures thinking level changes during a session are persisted
and correctly restored on --continue.
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