v1.11: opencode-style compaction port

- compaction.ts: usable/isOverflow/estimate/turns/select/buildPrompt/process
- compaction-prompt.ts: SUMMARY_TEMPLATE verbatim from opencode
- schema: messages.{compacted_at,summary,tail_start_id} + chats.needs_compaction
- inference: auto-trigger on overflow, pre-fetch compaction before next turn
- /compact slash command rewired to new path
- WS: chat_status working/idle around compaction + compacted frame
- frontend: SummaryCard + sonner toast on compacted
- 24 unit tests for pure functions

apps/web/src/hooks/useSessionStream.ts

@@ -1,5 +1,7 @@
 import { useEffect, useRef, useState } from 'react';
+import { toast } from 'sonner';
 import type { Message, WsFrame } from '@/api/types';
+import { api } from '@/api/client';
 import { sessionEvents } from './sessionEvents';
 
 // session_renamed frame removed from WsFrame — it was declared but never
@@ -161,6 +163,12 @@ function applyFrame(state: State, frame: WsFrame): State {
         : state.messages;
       return { ...state, messages: next, error: frame.error };
     }
+    case 'compacted': {
+      // v1.11: side effects (refetch + toast) live in ws.onmessage; the
+      // reducer just no-ops so TS exhaustiveness is satisfied without
+      // duplicating async work inside a synchronous reducer.
+      return state;
+    }
   }
 }
 
@@ -196,6 +204,25 @@ export function useSessionStream(sessionId: string | undefined) {
       ws.onmessage = (ev) => {
         try {
           const frame = JSON.parse(typeof ev.data === 'string' ? ev.data : '') as WsFrame;
+          // v1.11: on a compaction completion, re-fetch the message list so
+          // the new summary row + the cohort of compacted_at-stamped older
+          // rows render correctly. We dispatch the fresh list as a synthetic
+          // 'snapshot' frame so the reducer's existing path handles state
+          // replacement (no need for a parallel "refetched" path).
+          // The toast is purely UX feedback; missing it would still leave
+          // the chat in a valid state.
+          if (frame.type === 'compacted') {
+            toast.success('Context compacted to free space');
+            void api.messages
+              .list(frame.session_id)
+              .then((messages) => {
+                setState((s) => applyFrame(s, { type: 'snapshot', messages }));
+              })
+              .catch((err: unknown) => {
+                console.warn('compacted refetch failed', err);
+              });
+            return;
+          }
           setState((s) => applyFrame(s, frame));
         } catch (err) {
           console.warn('bad ws frame', err);