feat: add cache-user-id toggle for Claude cloaking

Default to generating a fresh random user_id per request instead of
reusing cached IDs. Add cache-user-id config option to opt in to the
previous caching behavior.

- Add CacheUserID field to CloakConfig
- Extract user_id cache logic to dedicated file
- Generate fresh user_id by default, cache only when enabled
- Add tests for both paths

internal/runtime/executor/user_id_cache.go

@@ -0,0 +1,89 @@
+package executor
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"sync"
+	"time"
+)
+
+type userIDCacheEntry struct {
+	value  string
+	expire time.Time
+}
+
+var (
+	userIDCache            = make(map[string]userIDCacheEntry)
+	userIDCacheMu          sync.RWMutex
+	userIDCacheCleanupOnce sync.Once
+)
+
+const (
+	userIDTTL                = time.Hour
+	userIDCacheCleanupPeriod = 15 * time.Minute
+)
+
+func startUserIDCacheCleanup() {
+	go func() {
+		ticker := time.NewTicker(userIDCacheCleanupPeriod)
+		defer ticker.Stop()
+		for range ticker.C {
+			purgeExpiredUserIDs()
+		}
+	}()
+}
+
+func purgeExpiredUserIDs() {
+	now := time.Now()
+	userIDCacheMu.Lock()
+	for key, entry := range userIDCache {
+		if !entry.expire.After(now) {
+			delete(userIDCache, key)
+		}
+	}
+	userIDCacheMu.Unlock()
+}
+
+func userIDCacheKey(apiKey string) string {
+	sum := sha256.Sum256([]byte(apiKey))
+	return hex.EncodeToString(sum[:])
+}
+
+func cachedUserID(apiKey string) string {
+	if apiKey == "" {
+		return generateFakeUserID()
+	}
+
+	userIDCacheCleanupOnce.Do(startUserIDCacheCleanup)
+
+	key := userIDCacheKey(apiKey)
+	now := time.Now()
+
+	userIDCacheMu.RLock()
+	entry, ok := userIDCache[key]
+	valid := ok && entry.value != "" && entry.expire.After(now) && isValidUserID(entry.value)
+	userIDCacheMu.RUnlock()
+	if valid {
+		userIDCacheMu.Lock()
+		entry = userIDCache[key]
+		if entry.value != "" && entry.expire.After(now) && isValidUserID(entry.value) {
+			entry.expire = now.Add(userIDTTL)
+			userIDCache[key] = entry
+			userIDCacheMu.Unlock()
+			return entry.value
+		}
+		userIDCacheMu.Unlock()
+	}
+
+	newID := generateFakeUserID()
+
+	userIDCacheMu.Lock()
+	entry, ok = userIDCache[key]
+	if !ok || entry.value == "" || !entry.expire.After(now) || !isValidUserID(entry.value) {
+		entry.value = newID
+	}
+	entry.expire = now.Add(userIDTTL)
+	userIDCache[key] = entry
+	userIDCacheMu.Unlock()
+	return entry.value
+}