feat(rag): pgvector knowledge base with API and LLM tools

- services/rag: knowledge_docs store — vector search via the shared
  embedder (HNSW index, graceful ILIKE fallback), list/delete scoped to
  the user, search intentionally global for the single-user deployment
- gateway: GET/POST /knowledge/docs, DELETE /knowledge/docs/:id,
  POST /knowledge/search
- plugins/knowledge: knowledge_search and knowledge_ingest tools so the
  persona can save and recall information mid-conversation
- main: migrations, store wiring, plugin registration

backend/plugins/knowledge/plugin_test.go

@@ -0,0 +1,68 @@
+package knowledge
+
+import (
+	"context"
+	"strings"
+	"testing"
+
+	"git.yeij.top/AskaEth/Cyrene_v2/services/rag"
+)
+
+type fakeStore struct {
+	docs []rag.Document
+}
+
+func (f *fakeStore) Ingest(_ context.Context, userID, title, content string) (*rag.Document, error) {
+	d := &rag.Document{ID: int64(len(f.docs) + 1), UserID: userID, Title: title, Content: content}
+	f.docs = append(f.docs, *d)
+	return d, nil
+}
+
+func (f *fakeStore) Search(_ context.Context, _, query string, topK int) ([]rag.Document, error) {
+	var out []rag.Document
+	for _, d := range f.docs {
+		if strings.Contains(d.Content, query) || strings.Contains(d.Title, query) {
+			out = append(out, d)
+		}
+	}
+	return out, nil
+}
+
+func TestIngestAndSearch(t *testing.T) {
+	p := New(&fakeStore{})
+	ctx := context.Background()
+
+	out, err := p.Execute(ctx, "knowledge_ingest", map[string]any{"title": "火锅攻略", "content": "楼下那家火锅店周三半价"})
+	if err != nil {
+		t.Fatalf("ingest: %v", err)
+	}
+	if !strings.Contains(out, "火锅攻略") {
+		t.Errorf("ingest output: %q", out)
+	}
+
+	out, err = p.Execute(ctx, "knowledge_search", map[string]any{"query": "火锅"})
+	if err != nil {
+		t.Fatalf("search: %v", err)
+	}
+	if !strings.Contains(out, "周三半价") {
+		t.Errorf("search output: %q", out)
+	}
+}
+
+func TestSearchEmpty(t *testing.T) {
+	p := New(&fakeStore{})
+	out, err := p.Execute(context.Background(), "knowledge_search", map[string]any{"query": "不存在"})
+	if err != nil || !strings.Contains(out, "没有相关内容") {
+		t.Errorf("out=%q err=%v", out, err)
+	}
+}
+
+func TestUnavailableStore(t *testing.T) {
+	p := New(nil)
+	if p.Enabled() {
+		t.Error("plugin should be disabled without store")
+	}
+	if _, err := p.Execute(context.Background(), "knowledge_search", map[string]any{"query": "x"}); err == nil {
+		t.Error("expected error with nil store")
+	}
+}