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>

docs/bench-example.json

@@ -0,0 +1,33 @@
+{
+  "description": "Example search-quality fixture — copy and adapt against your own corpus",
+  "top": 10,
+  "queries": [
+    {
+      "id": "manual-lookup",
+      "query": "how do I check the oil level",
+      "doc_type": "pdf",
+      "relevant": [
+        {"document_id": 2077},
+        {"title_contains": "owner's manual"}
+      ],
+      "notes": "Selectors are OR-matched; each counts as one relevant document. Exactly one of document_id / source_path / title_contains per selector."
+    },
+    {
+      "id": "infra-runbook",
+      "query": "restart the reverse proxy after certificate renewal",
+      "tags": ["ops"],
+      "relevant": [
+        {"source_path": "/data/notes/proxy-runbook.md"}
+      ]
+    },
+    {
+      "id": "note-recall",
+      "query": "what did we decide about the backup retention window",
+      "top": 5,
+      "relevant": [
+        {"title_contains": "backup retention"}
+      ],
+      "notes": "Per-query top overrides the fixture-level default; the --top flag overrides both."
+    }
+  ]
+}