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/automation/index.ts

@@ -1,2 +1,4 @@
 export { CronScheduler } from './cron.js';
 export { WebhookHandler } from './webhooks.js';
+export { HeartbeatMonitor, parseInterval } from './heartbeat.js';
+export type { HeartbeatResult, HeartbeatDeps, CheckResult } from './heartbeat.js';