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/context.go

@@ -36,9 +36,10 @@ type ContextItem struct {
 }
 
 type ProjectContextOutput struct {
-	Project thoughttypes.Project `json:"project"`
-	Context string               `json:"context"`
-	Items   []ContextItem        `json:"items"`
+	Project       thoughttypes.Project `json:"project"`
+	Context       string               `json:"context"`
+	Items         []ContextItem        `json:"items"`
+	RetrievalMode string               `json:"retrieval_mode,omitempty"`
 }
 
 func NewContextTool(db *store.DB, embeddings *ai.EmbeddingRunner, search config.SearchConfig, sessions *session.ActiveProjects) *ContextTool {
@@ -70,12 +71,14 @@ func (t *ContextTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in P
 		})
 	}
 
+	var retrievalMode string
 	query := strings.TrimSpace(in.Query)
 	if query != "" {
-		semantic, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, t.search.DefaultThreshold, &project.NumericID, nil)
+		semantic, mode, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, t.search.DefaultThreshold, &project.NumericID, nil)
 		if err != nil {
 			return nil, ProjectContextOutput{}, err
 		}
+		retrievalMode = mode
 		for _, result := range semantic {
 			key := fmt.Sprint(result.ID)
 			if _, ok := seen[key]; ok {
@@ -100,8 +103,9 @@ func (t *ContextTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in P
 	_ = t.store.TouchProject(ctx, project.NumericID)
 
 	return nil, ProjectContextOutput{
-		Project: *project,
-		Context: contextBlock,
-		Items:   items,
+		Project:       *project,
+		Context:       contextBlock,
+		Items:         items,
+		RetrievalMode: retrievalMode,
 	}, nil
 }