feat(memory): add experimental qmd search backend

docs/plans/2026-02-16-qmd-backend-checklist.md

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+# QMD Backend Checklist
+
+Date: 2026-02-16
+Status: completed
+
+## Scope
+
+- Add an experimental QMD (query markdown database) backend for `memory.search`.
+- Enable config-driven backend selection between hybrid embeddings, QMD, and keyword fallback.
+- Update docs and plan state.
+
+## Completed
+
+- Added `memory.qmd` config schema in `src/config/schema.ts`:
+  - `enabled` (default `false`)
+  - `top_k` (default `8`)
+  - `min_score` (default `0.15`)
+- Implemented `QmdSearch` backend in `src/memory/qmd-search.ts`:
+  - heading-aware scoring
+  - token overlap + phrase bonus ranking
+  - session namespace recency boost
+- Wired backend selection in `src/daemon/memory.ts`:
+  - embedding enabled -> hybrid backend
+  - else if qmd enabled -> QMD backend
+  - else keyword-only search
+- Generalized memory search tool wiring:
+  - introduced shared backend interface for `memory.search`
+  - updated memory tool factory to accept any backend implementing `search(query, topK?)`
+- Updated docs:
+  - README memory section now documents QMD config and backend precedence.
+  - OpenClaw gap docs updated to mark QMD backend as implemented.
+- Added tests:
+  - `src/memory/qmd-search.test.ts`
+  - `src/config/schema.test.ts` coverage for `memory.qmd`
+
+## Verification
+
+- `pnpm test:run src/config/schema.test.ts src/memory/qmd-search.test.ts`
+- `pnpm typecheck`