feat(tui): wire /rewind through command.dispatch + prefill payload (#21910)

Adds the TUI half of the /rewind feature so the Ink terminal UI gets
the same affordance as the prompt_toolkit CLI.

Python side (tui_gateway/server.py):
- /rewind added to _PENDING_INPUT_COMMANDS so slash.exec rejects it
  and the TUI falls through to command.dispatch (the only path with
  access to live session state + memory hooks).
- New command.dispatch branch for name == "rewind":
  v1 auto-picks the most recent user turn (Claude-Code-style single-
  step undo), calls SessionDB.rewind_to_message, refreshes the
  in-memory history, fires _memory_manager.on_session_switch with
  rewound=True, and returns the new "prefill" payload.
- A dedicated picker overlay (multi-step rewind) is tracked as a
  follow-up to #21910.

TS side (ui-tui/src/):
- New "prefill" variant on CommandDispatchResponse + asCommandDispatch
  validator. Mirrors "send" but does NOT auto-submit; the client drops
  the message into the composer for editing.
- createSlashHandler renders the optional notice via sys() and calls
  ctx.composer.setInput(d.message), letting the user edit-and-resubmit
  the rewound turn — the core UX promised by the issue.

Tests:
- 7 new tui_gateway tests covering prefill payload shape, in-memory
  history truncation, DB soft-delete, memory-provider notification
  (rewound=True), busy-session refusal, missing-session error, and
  registry placement in _PENDING_INPUT_COMMANDS.
- Extended asCommandDispatch vitest covering the new prefill variant
  (with + without notice, and rejection of malformed payloads).

Out of scope for v1 (tracked as #21910 follow-up):
- Dedicated picker overlay in Ink (the multi-step rewind UI). v1 auto-
  picks the most recent user turn, matching the most common case.
- Gateway platforms (Telegram, Discord, etc.) — issue scopes v1 to
  CLI + TUI only.

ui-tui/src/__tests__/asCommandDispatch.test.ts

@@ -15,6 +15,15 @@ describe('asCommandDispatch', () => {
       type: 'send',
       message: 'hello world'
     })
+    expect(asCommandDispatch({ type: 'prefill', message: 'edit me' })).toEqual({
+      type: 'prefill',
+      message: 'edit me'
+    })
+    expect(asCommandDispatch({ type: 'prefill', message: 'edit me', notice: '↶ rewound' })).toEqual({
+      type: 'prefill',
+      message: 'edit me',
+      notice: '↶ rewound'
+    })
   })
 
   it('rejects malformed payloads', () => {
@@ -23,5 +32,7 @@ describe('asCommandDispatch', () => {
     expect(asCommandDispatch({ type: 'skill', name: 1 })).toBeNull()
     expect(asCommandDispatch({ type: 'send' })).toBeNull()
     expect(asCommandDispatch({ type: 'send', message: 42 })).toBeNull()
+    expect(asCommandDispatch({ type: 'prefill' })).toBeNull()
+    expect(asCommandDispatch({ type: 'prefill', message: 42 })).toBeNull()
   })
 })

ui-tui/src/app/createSlashHandler.ts

@@ -119,6 +119,19 @@ export function createSlashHandler(ctx: SlashHandlerContext): (cmd: string) => b
               }
               return d.message?.trim() ? send(d.message) : sys(`/${parsed.name}: empty message`)
             }
+
+            if (d.type === 'prefill') {
+              // /rewind returns prefill: drop the chosen text into the
+              // composer so the user can edit and resubmit, instead of
+              // submitting it immediately like 'send'.
+              if (d.notice?.trim()) {
+                sys(d.notice)
+              }
+              if (d.message) {
+                ctx.composer.setInput(d.message)
+              }
+              return
+            }
           })
           .catch(guardedErr)
       })

ui-tui/src/gatewayTypes.ts

@@ -48,6 +48,7 @@ export type CommandDispatchResponse =
   | { target: string; type: 'alias' }
   | { message?: string; name: string; type: 'skill' }
   | { message: string; notice?: string; type: 'send' }
+  | { message: string; notice?: string; type: 'prefill' }
 
 // ── Config ───────────────────────────────────────────────────────────
 

ui-tui/src/lib/rpc.ts

@@ -34,6 +34,14 @@ export const asCommandDispatch = (value: unknown): CommandDispatchResponse | nul
     }
   }
 
+  if (t === 'prefill' && typeof o.message === 'string') {
+    return {
+      type: 'prefill',
+      message: o.message,
+      notice: typeof o.notice === 'string' ? o.notice : undefined,
+    }
+  }
+
   return null
 }