fix(8.0): neural.clusters()/similarity must request vectors from get()

brain.get() returns metadata only by default (includeVectors: false) for speed;
the vector is fetched via get(id, { includeVectors: true }). The neural API's
clustering, similarity, and vector-conversion helpers called get() without it,
so every entity came back with vector: [] — _getItemsWithVectors filtered them
all out, leaving k-means++ to dereference an empty array ("Cannot read
properties of undefined (reading 'vector')"). neural.clusters() was effectively
non-functional, and similarity-by-id silently scored 0.

- Pass { includeVectors: true } at the 8 vector-consuming get() call sites
  (similarity, _convertToVector, _getItemsWith{Vectors,Metadata}, cluster
  membership + quality scoring).
- Guard k-means against an empty item set (return an empty result, never crash).

"cluster entities semantically" passes. (includeVFS clustering + vfs.move have a
separate VFS-vector-dimension issue, tracked in .strategy.)

src/neural/improvedNeuralAPI.ts

@@ -1076,6 +1076,12 @@ export class ImprovedNeuralAPI {
     // Get vectors for all items using existing infrastructure
     const itemsWithVectors = await this._getItemsWithVectors(itemIds)
 
+    // No items carried a usable vector (e.g. all vectorless) — return an empty
+    // result rather than letting centroid init dereference an empty array.
+    if (itemsWithVectors.length === 0) {
+      return this._createEmptyResult(startTime, 'kmeans')
+    }
+
     // Determine optimal k
     const k = options.maxClusters || Math.min(
       Math.floor(Math.sqrt(itemsWithVectors.length / 2)),
@@ -1448,8 +1454,8 @@ export class ImprovedNeuralAPI {
   // Similarity implementation methods
   private async _similarityById(id1: string, id2: string, options: SimilarityOptions): Promise<number | SimilarityResult> {
     // Get vectors for both items
-    const item1 = await this.brain.get(id1)
+    const item1 = await this.brain.get(id1, { includeVectors: true })
-    const item2 = await this.brain.get(id2)
+    const item2 = await this.brain.get(id2, { includeVectors: true })
     
     if (!item1 || !item2) {
       return 0
@@ -1520,7 +1526,7 @@ export class ImprovedNeuralAPI {
     if (this._isVector(input)) {
       return input
     } else if (this._isId(input)) {
-      const item = await this.brain.get(input)
+      const item = await this.brain.get(input, { includeVectors: true })
       return item?.vector || []
     } else if (typeof input === 'string') {
       return await this.brain.embed(input)
@@ -1795,7 +1801,7 @@ export class ImprovedNeuralAPI {
   private async _getItemsWithMetadata(itemIds: string[]): Promise<ItemWithMetadata[]> {
     const items = await Promise.all(
       itemIds.map(async id => {
-        const noun = await this.brain.get(id)
+        const noun = await this.brain.get(id, { includeVectors: true })
         if (!noun) {
           return null
         }
@@ -2087,7 +2093,7 @@ export class ImprovedNeuralAPI {
   private async _getItemsWithVectors(itemIds: string[]): Promise<Array<{id: string, vector: number[]}>> {
     const items = await Promise.all(
       itemIds.map(async id => {
-        const noun = await this.brain.get(id)
+        const noun = await this.brain.get(id, { includeVectors: true })
         return {
           id,
           vector: noun?.vector || []
@@ -3208,7 +3214,7 @@ export class ImprovedNeuralAPI {
 
   private async _calculateItemToClusterSimilarity(itemId: string, cluster: SemanticCluster): Promise<number> {
     // Calculate similarity between an item and a cluster centroid
-    const item = await this.brain.get(itemId)
+    const item = await this.brain.get(itemId, { includeVectors: true })
     if (!item || !item.vector || !cluster.centroid) {
       return 0 // No similarity if vectors missing
     }
@@ -3234,7 +3240,7 @@ export class ImprovedNeuralAPI {
     // Get all member vectors
     const memberVectors: Vector[] = []
     for (const memberId of cluster.members) {
-      const member = await this.brain.get(memberId)
+      const member = await this.brain.get(memberId, { includeVectors: true })
       if (member && member.vector) {
         memberVectors.push(member.vector)
       }
@@ -3630,7 +3636,7 @@ export class ImprovedNeuralAPI {
       if (clusters.length === 0) break
       
       try {
-        const noun = await this.brain.get(itemId)
+        const noun = await this.brain.get(itemId, { includeVectors: true })
         const itemVector = noun?.vector || []
         if (itemVector.length === 0) continue