fix(kilo-sessions): cache undefined values in in-flight cache

Track entry presence separately from the cached value so `undefined`
results are treated as valid cache hits within the TTL.

packages/opencode/src/kilo-sessions/inflight-cache.ts

@@ -1,6 +1,7 @@
 type Entry<T> = {
   at: number
   value: T | undefined
+  has: boolean
   inflight: Promise<T> | undefined
 }
 
@@ -11,7 +12,8 @@ export function withInFlightCache<T>(key: string, ttlMs: number, cb: () => Promi
   const existing = store.get(key) as Entry<T> | undefined
 
   if (existing) {
-    if (existing.value !== undefined && now - existing.at < ttlMs) return Promise.resolve(existing.value)
+    // Allow caching `undefined` by tracking presence separately.
+    if (existing.has && now - existing.at < ttlMs) return Promise.resolve(existing.value as T)
     if (existing.inflight && now - existing.at < ttlMs) return existing.inflight
   }
 
@@ -19,16 +21,19 @@ export function withInFlightCache<T>(key: string, ttlMs: number, cb: () => Promi
     ? {
         at: now,
         value: existing.value,
+        has: existing.has,
         inflight: undefined,
       }
     : {
         at: now,
         value: undefined,
+        has: false,
         inflight: undefined,
       }
 
   const task = cb().then((value) => {
     next.value = value
+    next.has = true
     return value
   })