feat: reset idle assistant sessions after one hour

When a conversation's assistant session has been idle longer than
runtime.acp.assistantSessionResetIdleMs (default 1h, 0 disables) and
its owner has no unfinished proposals, the next inbound message starts
a fresh session instead of resuming. Binding lastActiveAt is persisted
per turn so the decision survives restarts.

src/core/durable-session-store.ts

@@ -14,6 +14,8 @@ export interface AssistantBinding {
   botFingerprint: string;
   createdAt: number;
   updatedAt: number;
+  // Refreshed on every assistant turn; drives the idle reset of long-inactive sessions.
+  lastActiveAt?: number;
 }
 
 interface StoreData {
@@ -90,6 +92,7 @@ export class DurableSessionStore {
     const binding = this.data.bindings[chatKey];
     if (!binding) return;
     binding.updatedAt = Date.now();
+    binding.lastActiveAt = binding.updatedAt;
     await this.persist();
   }
 
@@ -148,7 +151,8 @@ function parseStoreData(value: unknown): StoreData {
       || typeof binding.assistantWorkspace !== "string"
       || typeof binding.botFingerprint !== "string"
       || typeof binding.createdAt !== "number"
-      || typeof binding.updatedAt !== "number") {
+      || typeof binding.updatedAt !== "number"
+      || (binding.lastActiveAt !== undefined && typeof binding.lastActiveAt !== "number")) {
       throw new Error(`invalid state v3 binding '${chatKey}'`);
     }
   }