Add namespace-scoped event stores for storage isolation

Add support for optional namespace prefixes on JetStreamEventStore streams
to enable complete namespace isolation at the storage level:

- Add Namespace field to JetStreamConfig
- Add NewJetStreamEventStoreWithNamespace convenience constructor
- Prefix stream names with sanitized namespace when configured
- Add GetNamespace and GetStreamName accessor methods
- Add unit tests for namespace functionality
- Document namespace-scoped stores in CLAUDE.md

The namespace prefix is sanitized (spaces, dots, wildcards converted to
underscores) and prepended to the stream name, ensuring events from one
namespace cannot be read from another namespace's store while maintaining
full backward compatibility for non-namespaced stores.

Closes #19

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

store/jetstream.go

@@ -23,6 +23,11 @@ type JetStreamConfig struct {
 	StreamRetention time.Duration
 	// ReplicaCount is the number of replicas for high availability (default: 1)
 	ReplicaCount int
+	// Namespace is an optional prefix for stream names to provide storage isolation.
+	// When set, the actual stream name becomes "{namespace}_{streamName}".
+	// Events in namespaced stores are completely isolated from other namespaces.
+	// Leave empty for backward-compatible non-namespaced behavior.
+	Namespace string
 }
 
 // DefaultJetStreamConfig returns the default configuration
@@ -48,6 +53,15 @@ func NewJetStreamEventStore(natsConn *nats.Conn, streamName string) (*JetStreamE
 	return NewJetStreamEventStoreWithConfig(natsConn, streamName, DefaultJetStreamConfig())
 }
 
+// NewJetStreamEventStoreWithNamespace creates a new JetStream-based event store with namespace isolation.
+// The namespace is prefixed to the stream name to ensure complete isolation at the storage level.
+// This is a convenience function; the same can be achieved by setting Namespace in JetStreamConfig.
+func NewJetStreamEventStoreWithNamespace(natsConn *nats.Conn, streamName string, namespace string) (*JetStreamEventStore, error) {
+	config := DefaultJetStreamConfig()
+	config.Namespace = namespace
+	return NewJetStreamEventStoreWithConfig(natsConn, streamName, config)
+}
+
 // NewJetStreamEventStoreWithConfig creates a new JetStream-based event store with custom configuration
 func NewJetStreamEventStoreWithConfig(natsConn *nats.Conn, streamName string, config JetStreamConfig) (*JetStreamEventStore, error) {
 	js, err := natsConn.JetStream()
@@ -63,10 +77,16 @@ func NewJetStreamEventStoreWithConfig(natsConn *nats.Conn, streamName string, co
 		config.ReplicaCount = DefaultReplicaCount
 	}
 
+	// Apply namespace prefix to stream name if provided
+	effectiveStreamName := streamName
+	if config.Namespace != "" {
+		effectiveStreamName = fmt.Sprintf("%s_%s", sanitizeSubject(config.Namespace), streamName)
+	}
+
 	// Create or update the stream
 	stream := &nats.StreamConfig{
-		Name:      streamName,
-		Subjects:  []string{fmt.Sprintf("%s.events.>", streamName), fmt.Sprintf("%s.snapshots.>", streamName)},
+		Name:      effectiveStreamName,
+		Subjects:  []string{fmt.Sprintf("%s.events.>", effectiveStreamName), fmt.Sprintf("%s.snapshots.>", effectiveStreamName)},
 		Storage:   nats.FileStorage,
 		Retention: nats.LimitsPolicy,
 		MaxAge:    config.StreamRetention,
@@ -80,12 +100,22 @@ func NewJetStreamEventStoreWithConfig(natsConn *nats.Conn, streamName string, co
 
 	return &JetStreamEventStore{
 		js:         js,
-		streamName: streamName,
+		streamName: effectiveStreamName,
 		config:     config,
 		versions:   make(map[string]int64),
 	}, nil
 }
 
+// GetNamespace returns the namespace configured for this store, or empty string if not namespaced.
+func (jes *JetStreamEventStore) GetNamespace() string {
+	return jes.config.Namespace
+}
+
+// GetStreamName returns the effective stream name (including namespace prefix if applicable).
+func (jes *JetStreamEventStore) GetStreamName() string {
+	return jes.streamName
+}
+
 // SaveEvent persists an event to JetStream.
 // Returns VersionConflictError if the event's version is not strictly greater
 // than the current latest version for the actor.