Close the EXT-8 remaining blocks: vector sharing, hostapi face, citation consistency

Why: EXT-8 closure needs the vector channel shared (not just fulltext), the /host/v1 knowledge face honoring the authorizer, and citation-ref-to-artifact consistency pinned as an acceptance test.
Evidence: TestRetrieveSharedVectorChannel (hash embedder: authorized shared vector hit retrievable, denied yields zero), TestSharedSearchFace (hostapi switches to SharedSearch when wired), TestSharedCitationRefDownloadable (shared hit -> Ref -> artifact content download for the authorized member); go test ./... 32/32 packages ok.
Impact: store.SearchKnowledgeVectorsShared; rag.Pipeline.RetrieveShared (own + shared fulltext + shared vector, RRF-merged); server searchKnowledge prefers RetrieveShared when the retriever implements it; hostapi Server.SharedSearch wired in the composition root through the same authorizer slot. Default behavior unchanged.
Refs: EXT-8 proposal acceptance; slices 1-2.

internal/hostapi/helper_test.go

@@ -0,0 +1,5 @@
+package hostapi
+
+import "encoding/json"
+
+func jsonUnmarshal(b []byte, v any) error { return json.Unmarshal(b, v) }

internal/hostapi/server.go

@@ -18,7 +18,10 @@ type Server struct {
 	Store    *store.Store
 	Dispatch func(ctx context.Context, pluginID, owner, commandID string, params map[string]any) error
 	Search   func(ctx context.Context, owner, text string) ([]core.RetrievalHit, error)
-	Complete func(ctx context.Context, owner, prompt string) (string, error)
+	// SharedSearch EXT-8 余项：非 nil 时 search 端点改用之（组合根传入
+	// 「本人结果 + 适配器放行的共享追加」组合；nil=现状仅本人）。
+	SharedSearch func(ctx context.Context, owner, text string) ([]core.RetrievalHit, error)
+	Complete     func(ctx context.Context, owner, prompt string) (string, error)
 }
 
 func (s *Server) Handler() http.Handler {
@@ -143,7 +146,12 @@ func (s *Server) search(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "retriever unavailable", http.StatusServiceUnavailable)
 		return
 	}
-	hits, err := s.Search(r.Context(), s.owner(r), r.URL.Query().Get("q"))
+	// EXT-8 余项：共享版可用时整体切换（本人+共享追加）。
+	fn := s.Search
+	if s.SharedSearch != nil {
+		fn = s.SharedSearch
+	}
+	hits, err := fn(r.Context(), s.owner(r), r.URL.Query().Get("q"))
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusBadGateway)
 		return

internal/hostapi/shared_search_test.go

@@ -0,0 +1,49 @@
+package hostapi
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	core "agent-system-core"
+)
+
+// EXT-8 余项：hostapi 检索面——SharedSearch 可用时整体切换（本人+适配器放行的
+// 共享追加）；不可用时维持现状（仅本人回调）。
+func TestSharedSearchFace(t *testing.T) {
+	own := []core.RetrievalHit{{Doc: core.KnowledgeDoc{Owner: "bob", Chunk: "本人命中"}}}
+	sharedHit := core.RetrievalHit{Doc: core.KnowledgeDoc{Owner: "alice", ProjectID: "prj-s", Chunk: "共享命中"}}
+
+	srv := &Server{Token: "t", Search: func(ctx context.Context, owner, text string) ([]core.RetrievalHit, error) {
+		return own, nil
+	}}
+	run := func() int {
+		rec := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/host/v1/knowledge/search?q=x", nil)
+		req.Header.Set("Authorization", "Bearer "+srv.Token)
+		srv.Handler().ServeHTTP(rec, req)
+		return rec.Code
+	}
+	if code := run(); code != http.StatusOK {
+		t.Fatalf("plain search: %d", code)
+	}
+
+	srv.SharedSearch = func(ctx context.Context, owner, text string) ([]core.RetrievalHit, error) {
+		return append(own, sharedHit), nil
+	}
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/host/v1/knowledge/search?q=x", nil)
+	req.Header.Set("Authorization", "Bearer "+srv.Token)
+	srv.Handler().ServeHTTP(rec, req)
+	if rec.Code != http.StatusOK {
+		t.Fatalf("shared search: %d %s", rec.Code, rec.Body.String())
+	}
+	var hits []core.RetrievalHit
+	if err := jsonUnmarshal(rec.Body.Bytes(), &hits); err != nil {
+		t.Fatal(err)
+	}
+	if len(hits) != 2 || hits[1].Doc.Owner != "alice" {
+		t.Fatalf("hits = %+v, want 本人+共享", hits)
+	}
+}