feat: add heartbeat monitor and vector memory search (Tier 2)

Heartbeat:
- HeartbeatMonitor with 5 checks: gateway, model, channels, memory, disk
- Configurable interval, failure threshold, notification channel
- Recovery notifications when health restores
- 25 new tests

Vector Memory Search:
- EmbeddingProvider interface with OpenAI, Gemini, Ollama, LlamaCpp backends
- SQLite-backed VectorStore with cosine similarity search
- Text chunker with paragraph-aware splitting and overlap
- HybridSearch merging keyword + vector results with configurable weight
- Background indexer with dirty-namespace tracking
- Graceful fallback to keyword search when embeddings unavailable
- 51 new tests

Config: automation.heartbeat + memory.embedding schema sections
Total: 950 tests passing, all types clean

src/memory/store.ts

@@ -40,6 +40,7 @@ export interface SearchResult {
  */
 export class MemoryStore {
   private _config: MemoryStoreConfig;
+  private _dirtyNamespaces: Set<string> = new Set();
 
   constructor(config: MemoryStoreConfig) {
     this._config = config;
@@ -88,6 +89,9 @@ export class MemoryStore {
       const separator = existing.length > 0 ? '\n' : '';
       writeFileSync(filePath, existing + separator + content, 'utf-8');
     }
+
+    // Mark namespace as needing re-indexing for vector search
+    this._dirtyNamespaces.add(namespace);
   }
 
   /**
@@ -147,6 +151,25 @@ export class MemoryStore {
     return this._scanDir(this._config.dir);
   }
 
+  /**
+   * Return namespaces that have been modified since last call, then clear
+   * the dirty set. Used by the background indexer to re-embed changed content.
+   */
+  getDirtyNamespaces(): string[] {
+    const dirty = Array.from(this._dirtyNamespaces);
+    this._dirtyNamespaces.clear();
+    return dirty;
+  }
+
+  /**
+   * Mark all existing namespaces as dirty (e.g. for initial full indexing).
+   */
+  markAllDirty(): void {
+    for (const ns of this.listNamespaces()) {
+      this._dirtyNamespaces.add(ns);
+    }
+  }
+
   /**
    * Build memory context suitable for injection into a system prompt.
    *