Make configuration values injectable rather than hardcoded

Add config structs with sensible defaults for tunable parameters:
- JetStreamConfig for stream retention (1 year) and replica count (1)
- HashRingConfig for virtual nodes per physical node (150)
- ShardConfig for shard count (1024) and replication factor (1)

Each component gets a new WithConfig constructor that accepts custom
configuration, while the original constructors continue to work with
defaults. Zero values in configs fall back to defaults for backward
compatibility.

Closes #38

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

cluster/manager.go

@@ -20,17 +20,17 @@ type VMRegistry interface {
 
 // ClusterManager coordinates distributed VM operations across the cluster
 type ClusterManager struct {
-	nodeID       string
-	nodes        map[string]*NodeInfo
-	nodeUpdates  chan NodeUpdate
-	shardMap     *ShardMap
-	hashRing     *ConsistentHashRing
-	election     *LeaderElection
-	natsConn     *nats.Conn
-	ctx          context.Context
-	mutex        sync.RWMutex
-	logger       *log.Logger
-	vmRegistry   VMRegistry // Interface to access local VMs
+	nodeID      string
+	nodes       map[string]*NodeInfo
+	nodeUpdates chan NodeUpdate
+	shardMap    *ShardMap
+	hashRing    *ConsistentHashRing
+	election    *LeaderElection
+	natsConn    *nats.Conn
+	ctx         context.Context
+	mutex       sync.RWMutex
+	logger      *log.Logger
+	vmRegistry  VMRegistry // Interface to access local VMs
 }
 
 // NewClusterManager creates a cluster coordination manager
@@ -328,4 +328,4 @@ func (cm *ClusterManager) GetShardMap() *ShardMap {
 		Nodes:      make(map[string]NodeInfo),
 		UpdateTime: cm.shardMap.UpdateTime,
 	}
-}
+}