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

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

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+# Memory v2 — Hybrid Search & Auto-Extract
+
+**Status:** Proposed
+**Epic:** memory-v2-hybrid-search
+**Depends on:** v2.8.0-fork-lifts (v1 memory already shipped)
+
+## Why
+
+v1 memory (shipped in v2.8.0-fork-lifts) provides file-based recall with keyword/tag matching injected into `system-prompt.ts`. It works but has three gaps:
+
+1. **Keyword-only recall misses semantic matches** — "indentation" won't match a memory entry titled "Code style: tabs vs spaces" unless the word "indentation" appears verbatim.
+2. **No auto-extraction** — memory files must be created manually. The LLM can't persist useful facts it discovers during conversation.
+3. **Flat search, no ranking** — all keyword matches are equally weighted. No relevance scoring or deduplication.
+
+v2 upgrades the retrieval layer while keeping the file-based storage format. No breaking changes to `.boocode/memory/` structure.
+
+## What Changes
+
+### Hybrid Search (high confidence)
+Replace keyword-only `rankByRelevance` with BM25 + embedding hybrid search. Use a tiny local embedding model (all-MiniLM-L6-v2 through ONNX runtime or a local subprocess) so there's no external API dependency.
+
+- **BM25** (already implementable without deps — term frequency + inverse document frequency scoring on the memory entries)
+- **Embedding** (local ONNX model, ~20MB, runs inference in ~5ms on CPU, produces 384-dim vectors)
+- **Weighted merge** (`score = 0.3 * bm25 + 0.7 * cosine`) — configurable ratio
+
+### Auto-Extract Agent Tool (medium confidence)
+A new `extract_memory` tool exposed to agents (not automatic — agent decides when to persist):
+
+- `extract_memory(topic, title, content, tags)` → writes a markdown entry
+- `search_memory(query)` → returns ranked memory entries (new tool, replaces raw injection)
+
+### In-Memory Embedding Cache (optional)
+Keep embeddings in an LRU map keyed by file mtime. Recompute only when files change. No DB migration needed.
+
+## Non-Goals
+- No vector database (SQLite FTS5 or in-memory BM25 suffice)
+- No automatic background extraction agent (agent must explicitly call `extract_memory`)
+- No changes to the `.boocode/memory/` file format
+- No Python dependencies — ONNX runtime is a Node.js native addon or subprocess