docs: add openspec proposals for memory v2 and orchestrator flow patterns

openspec/changes/memory-v2-hybrid-search/design.md

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+# Memory v2 — Design
+
+## Architecture
+
+```
+                     ┌─────────────────────────┐
+                     │    system-prompt.ts      │
+                     │  (inject memory block)   │
+                     └────────┬────────────────┘
+                              │
+                    ┌─────────▼──────────┐
+                    │    memory/recall.ts │
+                    │  (renamed to query) │
+                    └─────────┬──────────┘
+                              │
+              ┌───────────────┼───────────────┐
+              │               │               │
+     ┌────────▼──────┐ ┌─────▼──────┐ ┌──────▼───────┐
+     │  BM25Ranker   │ │ EmbedCache │ │ CosineRanker │
+     │  (stateless)  │ │ (LRU map)  │ │  (ONNX)      │
+     └───────────────┘ └────────────┘ └──────────────┘
+```
+
+## Module Changes
+
+### `apps/server/src/services/memory/` — new/changed files
+
+| File | Change |
+|------|--------|
+| `recall.ts` | Replace `rankByRelevance` with hybrid `rankByHybrid(query, entries)` |
+| `embeddings.ts` | **New** — ONNX model loader + `embed(texts: string[]): number[][]` |
+| `bm25.ts` | **New** — BM25 scorer with `score(query, doc): number` |
+| `ranker.ts` | **New** — weighted merge of BM25 + cosine scores |
+| `entries.ts` | Add `serializeForEmbedding(entry): string` helper |
+
+### Embedding Model
+
+- Model: `all-MiniLM-L6-v2` (384-dim, ~23MB ONNX)
+- Runtime: `onnxruntime-node` npm package or subprocess via `node:child_process`
+- Cache: `Map<string, { embedding: number[], mtime: number }>` in-memory, cleared on process restart
+- Fallback: BM25-only when model file is missing
+
+### Agent Tools (new)
+
+| Tool | Description |
+|------|-------------|
+| `extract_memory(topic, title, content, tags?)` | Persists a memory entry. Topic must be one of project/user/reference |
+| `search_memory(query)` | Returns up to 10 ranked memory entries matching the query. Replaces blind injection |
+
+### Scoring Formula
+
+```
+score = (BM25_score * 0.3) + (cosine_similarity * 0.7)
+```
+
+Both normalized to [0,1] before merging. Entries below threshold (0.15) are excluded.
+
+## Rollback
+
+Set `MEMORY_SEARCH=keyword` env var to fall back to the v1 keyword-only path. Default is `hybrid`.