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.go

@@ -0,0 +1,93 @@
+// Package knowledge exposes the knowledge base as LLM tools.
+package knowledge
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"git.yeij.top/AskaEth/Cyrene_v2/internal/plugin"
+	"git.yeij.top/AskaEth/Cyrene_v2/services/rag"
+)
+
+// Store is the subset of rag.Store used by the tools.
+type Store interface {
+	Ingest(ctx context.Context, userID, title, content string) (*rag.Document, error)
+	Search(ctx context.Context, userID, query string, topK int) ([]rag.Document, error)
+}
+
+type Plugin struct{ store Store }
+
+func New(store Store) *Plugin { return &Plugin{store: store} }
+
+func (p *Plugin) Name() string        { return "knowledge" }
+func (p *Plugin) Version() string     { return "1.0.0" }
+func (p *Plugin) Description() string { return "知识库检索与写入" }
+func (p *Plugin) Init(_ context.Context, _ plugin.PluginConfig) error { return nil }
+func (p *Plugin) Close() error        { return nil }
+func (p *Plugin) Enabled() bool       { return p.store != nil }
+
+func (p *Plugin) Tools() []plugin.ToolDefinition {
+	return []plugin.ToolDefinition{
+		{
+			Name:        "knowledge_search",
+			Description: "Search the knowledge base for relevant documents.",
+			Parameters: map[string]any{
+				"type": "object",
+				"properties": map[string]any{
+					"query": map[string]string{"type": "string", "description": "Search query"},
+				},
+				"required": []string{"query"},
+			},
+		},
+		{
+			Name:        "knowledge_ingest",
+			Description: "Save a piece of information into the knowledge base for later retrieval.",
+			Parameters: map[string]any{
+				"type": "object",
+				"properties": map[string]any{
+					"title":   map[string]string{"type": "string", "description": "Document title"},
+					"content": map[string]string{"type": "string", "description": "Document content"},
+				},
+				"required": []string{"title", "content"},
+			},
+		},
+	}
+}
+
+func (p *Plugin) Execute(ctx context.Context, toolName string, params map[string]any) (string, error) {
+	if p.store == nil {
+		return "", fmt.Errorf("knowledge store unavailable")
+	}
+	switch toolName {
+	case "knowledge_search":
+		query, _ := params["query"].(string)
+		if query == "" {
+			return "", fmt.Errorf("missing query")
+		}
+		docs, err := p.store.Search(ctx, "", query, 5)
+		if err != nil {
+			return "", err
+		}
+		if len(docs) == 0 {
+			return "知识库中没有相关内容", nil
+		}
+		var sb strings.Builder
+		for i, d := range docs {
+			fmt.Fprintf(&sb, "%d. 《%s》\n%s\n\n", i+1, d.Title, d.Content)
+		}
+		return sb.String(), nil
+
+	case "knowledge_ingest":
+		title, _ := params["title"].(string)
+		content, _ := params["content"].(string)
+		if title == "" || content == "" {
+			return "", fmt.Errorf("missing title or content")
+		}
+		if _, err := p.store.Ingest(ctx, "", title, content); err != nil {
+			return "", err
+		}
+		return fmt.Sprintf("已收录《%s》", title), nil
+	}
+	return "", fmt.Errorf("unknown tool %q", toolName)
+}