feat(tools): expose retrieval_mode in query-based tool responses

Add a retrieval_mode field ("semantic" or "text") to the output of all
five query-driven tools so callers can distinguish vector search from
Postgres full-text fallback without inspecting server logs.

Tools updated: search_thoughts, recall_context, get_project_context,
summarize_thoughts, and related_thoughts (semantic neighbours only;
omitempty so field is absent when include_semantic is false or no query
is provided for the non-mandatory-query tools).

The shared semanticSearch helper now returns the mode as a third return
value. All callers updated; fixes two compile errors left by the
previous worker (summarize.go and links.go were not capturing the new
return).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/tools/summarize.go

@@ -28,8 +28,9 @@ type SummarizeInput struct {
 }
 
 type SummarizeOutput struct {
-	Summary string `json:"summary"`
-	Count   int    `json:"count"`
+	Summary       string `json:"summary"`
+	Count         int    `json:"count"`
+	RetrievalMode string `json:"retrieval_mode,omitempty"`
 }
 
 func NewSummarizeTool(db *store.DB, embeddings *ai.EmbeddingRunner, metadata *ai.MetadataRunner, search config.SearchConfig, sessions *session.ActiveProjects) *SummarizeTool {
@@ -47,15 +48,17 @@ func (t *SummarizeTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in
 	lines := make([]string, 0, limit)
 	count := 0
 
+	var retrievalMode string
 	if query != "" {
 		var projectID *int64
 		if project != nil {
 			projectID = &project.NumericID
 		}
-		results, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, t.search.DefaultThreshold, projectID, nil)
+		results, mode, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, t.search.DefaultThreshold, projectID, nil)
 		if err != nil {
 			return nil, SummarizeOutput{}, err
 		}
+		retrievalMode = mode
 		for i, result := range results {
 			lines = append(lines, thoughtContextLine(i, result.Content, result.Metadata, result.Similarity))
 		}
@@ -85,5 +88,5 @@ func (t *SummarizeTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in
 		_ = t.store.TouchProject(ctx, project.NumericID)
 	}
 
-	return nil, SummarizeOutput{Summary: summary, Count: count}, nil
+	return nil, SummarizeOutput{Summary: summary, Count: count, RetrievalMode: retrievalMode}, nil
 }