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/search.go

@@ -28,7 +28,8 @@ type SearchInput struct {
 }
 
 type SearchOutput struct {
-	Results []thoughttypes.SearchResult `json:"results"`
+	Results       []thoughttypes.SearchResult `json:"results"`
+	RetrievalMode string                      `json:"retrieval_mode,omitempty"`
 }
 
 func NewSearchTool(db *store.DB, embeddings *ai.EmbeddingRunner, search config.SearchConfig, sessions *session.ActiveProjects) *SearchTool {
@@ -55,10 +56,10 @@ func (t *SearchTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in Se
 		_ = t.store.TouchProject(ctx, project.NumericID)
 	}
 
-	results, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, threshold, projectID, nil)
+	results, mode, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, threshold, projectID, nil)
 	if err != nil {
 		return nil, SearchOutput{}, err
 	}
 
-	return nil, SearchOutput{Results: results}, nil
+	return nil, SearchOutput{Results: results, RetrievalMode: mode}, nil
 }