* feat: add plan mode with interactive questions UI
Add generate_plan tool, structured plan types, and an OpenCode-style
inline questions panel with tabbed navigation, number-key shortcuts,
custom text answers, and a review/confirm tab.
* fix: plan-to-agent handover, mode-scoped tools, and UI polish
- Carry plan context forward when switching plan→agent mode
- Restrict generate_plan tool to plan mode only
- Stamp modeColor on all message types for correct per-message colors
- Sanitize model-echoed role labels and raw JSON from content
- Truncate tool result labels to single lines
- Single-word tab labels for plan questions
- Always show edit/write file results with diff
- Clear agent history on mode switch
