[P2] Plan mode dialog (EnterPlanMode / ExitPlanMode)

[quangdang46]

Plan Mode Dialog

Priority: P2 · Effort: 0.5 day

Current

No dialog when entering/exiting Plan mode. Mode just switches silently.

Target (Claude Code parity)

• EnterPlanMode: shows scope + reason for entering plan mode
• ExitPlanMode: shows changes made during plan mode

Implementation

1. Add draw_enter_plan_mode_dialog() in ui_overlays.rs
2. Add draw_exit_plan_mode_dialog() in ui_overlays.rs
3. Wire into Alt+P cycle or /permissions plan (input.rs)

Acceptance Criteria

• Dialog shown when entering plan mode via Alt+P
• Dialog explains that writes will be denied
• Dialog has Confirm/Cancel options