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/__tests__/destructive.test.ts

@@ -3,6 +3,10 @@ import { describe, expect, it } from 'vitest'
 import { CONFIRM_WINDOW_MS, createDestructiveGate } from '../domain/destructive.js'
 
 describe('createDestructiveGate', () => {
+  it('uses a generous default window so real humans can retype (#4069)', () => {
+    expect(CONFIRM_WINDOW_MS).toBeGreaterThanOrEqual(15_000)
+  })
+
   it('first request is not confirmed — it arms the gate', () => {
     const g = createDestructiveGate()
     expect(g.request('clear', 0)).toBe(false)
@@ -11,7 +15,7 @@ describe('createDestructiveGate', () => {
   it('second request within window with same key is confirmed', () => {
     const g = createDestructiveGate()
     g.request('clear', 0)
-    expect(g.request('clear', 2_500)).toBe(true)
+    expect(g.request('clear', CONFIRM_WINDOW_MS - 1)).toBe(true)
   })
 
   it('second request outside the window re-arms and is not confirmed', () => {
@@ -20,6 +24,15 @@ describe('createDestructiveGate', () => {
     expect(g.request('clear', CONFIRM_WINDOW_MS + 1)).toBe(false)
   })
 
+  it('armed() reports the pending key while fresh, null otherwise', () => {
+    const g = createDestructiveGate(100)
+    expect(g.armed()).toBe(null)
+    g.request('clear')
+    expect(g.armed()).toBe('clear')
+    g.reset()
+    expect(g.armed()).toBe(null)
+  })
+
   it('different key re-arms the gate, does not confirm', () => {
     const g = createDestructiveGate()
     g.request('clear', 0)