Add reranking, RRF fusion, bench harness, tag contexts, and data ingestion

Implements five of the six enhancements from docs/kb-enhancements-proposal.htm,
closing the retrieval-quality gap identified in the qmd review.

- Cross-encoder reranking: new kb/reranker.py loads an optional reranking
  model at startup (KB_RERANK_ENABLED, KB_RERANKER_MODEL,
  KB_RERANK_CANDIDATES). Search degrades gracefully to plain hybrid
  retrieval when the model is absent. Exposed via a "rerank" block in
  /status, a rerank flag on search, and --no-rerank in the CLI.
- RRF rank fusion: FTS and vector lists now merge by reciprocal rank
  fusion with a top-rank bonus, replacing the old score blend. Scores are
  comparable across queries.
- Bench harness and explain traces: kb bench runs a query fixture against
  each backend and reports precision@k, recall and MRR. --explain returns a
  per-result score breakdown.
- Tag context descriptions: tags carry an optional one-line description
  (kb tag-describe), returned as tag_contexts with search results. Adds a
  tags.description column migration.
- Structured data ingestion: .json/.yaml/.toml files ingest as text via the
  new "data" doc type, pretty-printing minified JSON before chunking.

Query expansion (proposal item 5) is deliberately left out pending bench
results. Requires engine v3.3.0.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

DEVELOPER.md

@@ -21,6 +21,35 @@ make build    # produces ./kb binary
 make all      # or cross-compile: dist/kb-{os}-{arch}
 ```
 
+## Running tests
+
+### Engine
+
+Engine tests run against SQLite (with sqlite-vec) and stub out the embedding
+model, so they only need lightweight dependencies — no torch/docling install:
+
+```bash
+uv venv /tmp/kb-test-venv
+uv pip install --python /tmp/kb-test-venv/bin/python pytest pytest-asyncio fastapi httpx sqlite-vec
+cd engine && /tmp/kb-test-venv/bin/python -m pytest
+```
+
+### Client
+
+```bash
+cd client && go test ./...
+```
+
+## Search-quality benchmarking
+
+`kb bench fixture.json` runs a fixture of queries with known-relevant documents
+against each backend (fts, vec, hybrid, hybrid+rerank) and reports precision@k,
+recall, and MRR. See `docs/bench-example.json` for the fixture format.
+
+Run a bench before and after any ranking change (RRF weights, reranker, model
+swap) and compare — keep a 20-30 query fixture against your real corpus outside
+the repo.
+
 ## Building and releasing
 
 Client and engine are versioned independently via `client/VERSION` and `engine/VERSION`. Each has its own release script and git tag prefix.
@@ -88,8 +117,9 @@ All endpoints are under `/api/v1/`. Requires `Authorization: Bearer <key>` heade
 | `GET` | `/documents/{id}/file` | Download original file |
 | `DELETE` | `/documents/{id}` | Remove a document (and stored file) |
 | `PUT` | `/documents/{id}/tags` | Add/remove tags |
-| `GET` | `/tags` | List all tags |
-| `GET` | `/status` | Engine status, GPU info, DB stats |
+| `GET` | `/tags` | List all tags (with descriptions) |
+| `PUT` | `/tags/{name}/description` | Set/clear a tag context description |
+| `GET` | `/status` | Engine status, GPU info, DB stats, rerank state |
 | `POST` | `/reindex` | Re-embed all chunks |
 | `POST` | `/bulk/delete` | Bulk delete documents by filter |
 | `POST` | `/bulk/tags` | Bulk add/remove tags by filter |