fix(tui): make /clear confirm window humane (3s → 30s, reset on other slash)

The 3s gate was too tight — users reading the prompt and retyping
consistently blow past it and get stuck in a loop ("press /clear
again within 3s" forever). Fixes:

- bump CONFIRM_WINDOW_MS 3_000 → 30_000
- drop the time number from the confirmation message to remove the
  pressure vibe: "press /clear again to confirm — starts a new session"
- reset the gate from createSlashHandler whenever any non-destructive
  slash command runs, so stale arming from 20s ago can't silently
  turn the next /clear into an unintended confirm
- export the gate + isDestructiveCommand helper for that wiring
- add armed() introspection method

Follow-up to #4069 / 3366714b.

ui-tui/src/app/slash/commands/core.ts

@@ -15,7 +15,11 @@ import { patchOverlayState } from '../../overlayStore.js'
 import { patchUiState } from '../../uiStore.js'
 import type { SlashCommand } from '../types.js'
 
-const destructiveGate = createDestructiveGate()
+export const destructiveGate = createDestructiveGate()
+
+const DESTRUCTIVE_COMMANDS = new Set(['clear', 'new'])
+
+export const isDestructiveCommand = (name: string) => DESTRUCTIVE_COMMANDS.has(name)
 
 const flagFromArg = (arg: string, current: boolean): boolean | null => {
   if (!arg) {
@@ -89,7 +93,7 @@ export const coreCommands: SlashCommand[] = [
       const label = cmd.startsWith('/new') ? '/new' : '/clear'
 
       if (!NO_CONFIRM_DESTRUCTIVE && !destructiveGate.request('clear')) {
-        return ctx.transcript.sys(`press ${label} again within 3s to confirm (starts a new session)`)
+        return ctx.transcript.sys(`press ${label} again to confirm — starts a new session`)
       }
 
       patchUiState({ status: 'forging session…' })