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/types.ts

@@ -29,6 +29,15 @@ export interface ApprovalReq {
   description: string
 }
 
+export interface ConfirmReq {
+  cancelLabel?: string
+  confirmLabel?: string
+  danger?: boolean
+  detail?: string
+  onConfirm: () => void
+  title: string
+}
+
 export interface ClarifyReq {
   choices: string[] | null
   question: string