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>

client/cmd/search_test.go

@@ -0,0 +1,67 @@
+package cmd
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"reflect"
+	"testing"
+)
+
+func TestSearchCmd_SendsDocTypeAndTagsList(t *testing.T) {
+	var captured map[string]interface{}
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/v1/search" {
+			t.Errorf("unexpected path: %s", r.URL.Path)
+		}
+		body, _ := io.ReadAll(r.Body)
+		if err := json.Unmarshal(body, &captured); err != nil {
+			t.Fatalf("failed to decode request body: %v", err)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"query":"q","results":[],"total_matches":0,"returned":0}`))
+	}))
+	defer server.Close()
+
+	var stdout bytes.Buffer
+	rootCmd.SetOut(&stdout)
+	rootCmd.SetArgs([]string{
+		"search", "oil level",
+		"--engine", server.URL,
+		"--format", "json",
+		"--type", "pdf",
+		"--tags", "manuals, ops,",
+	})
+
+	if err := rootCmd.Execute(); err != nil {
+		t.Fatalf("search command failed: %v", err)
+	}
+
+	if captured == nil {
+		t.Fatal("no request captured by test server")
+	}
+	if got := captured["doc_type"]; got != "pdf" {
+		t.Errorf("expected doc_type=pdf in body, got %v (full body: %v)", got, captured)
+	}
+	if _, present := captured["type"]; present {
+		t.Error("body must not contain legacy 'type' key")
+	}
+	tags, ok := captured["tags"].([]interface{})
+	if !ok {
+		t.Fatalf("expected tags to be a JSON array, got %T (%v)", captured["tags"], captured["tags"])
+	}
+	want := []interface{}{"manuals", "ops"}
+	if !reflect.DeepEqual(tags, want) {
+		t.Errorf("expected tags %v, got %v", want, tags)
+	}
+}
+
+func TestSplitTags(t *testing.T) {
+	got := splitTags(" a ,b,, c ")
+	want := []string{"a", "b", "c"}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("splitTags: expected %v, got %v", want, got)
+	}
+}