feat(tui): replace /clear double-press gate with a proper confirm overlay

The time-window gate felt wrong — users would hit /clear, read the
prompt, retype, and consistently blow past the window. Swapping to a
real yes/no overlay that blocks input like the existing Approval and
Clarify prompts.

- add ConfirmReq type + OverlayState.confirm + $isBlocked coverage
- ConfirmPrompt component (prompts.tsx): cancel row on top as the
  default, danger-coloured confirm row on the bottom, Y/N hotkeys,
  Enter on default = cancel, Esc/Ctrl+C cancel
- wire into PromptZone (appOverlays.tsx)
- /clear + /new now push onto the overlay instead of arming a timer
- HERMES_TUI_NO_CONFIRM=1 still skips the prompt for scripting
- drop the destructiveGate + createSlashHandler reset wiring
  (destructive.ts and its tests removed)

Refs #4069.

ui-tui/src/app/interfaces.ts

@@ -10,6 +10,7 @@ import type {
   ActivityItem,
   ApprovalReq,
   ClarifyReq,
+  ConfirmReq,
   DetailsMode,
   Msg,
   PanelSection,
@@ -53,6 +54,7 @@ export interface GatewayProviderProps {
 export interface OverlayState {
   approval: ApprovalReq | null
   clarify: ClarifyReq | null
+  confirm: ConfirmReq | null
   modelPicker: boolean
   pager: null | PagerState
   picker: boolean