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/recall.go

@@ -27,8 +27,9 @@ type RecallInput struct {
 }
 
 type RecallOutput struct {
-	Context string        `json:"context"`
-	Items   []ContextItem `json:"items"`
+	Context       string        `json:"context"`
+	Items         []ContextItem `json:"items"`
+	RetrievalMode string        `json:"retrieval_mode,omitempty"`
 }
 
 func NewRecallTool(db *store.DB, embeddings *ai.EmbeddingRunner, search config.SearchConfig, sessions *session.ActiveProjects) *RecallTool {
@@ -53,7 +54,7 @@ func (t *RecallTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in Re
 		projectID = &project.NumericID
 	}
 
-	semantic, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, t.search.DefaultThreshold, projectID, nil)
+	semantic, mode, err := semanticSearch(ctx, t.store, t.embeddings, t.search, query, limit, t.search.DefaultThreshold, projectID, nil)
 	if err != nil {
 		return nil, RecallOutput{}, err
 	}
@@ -100,7 +101,8 @@ func (t *RecallTool) Handle(ctx context.Context, req *mcp.CallToolRequest, in Re
 	}
 
 	return nil, RecallOutput{
-		Context: formatContextBlock(header, lines),
-		Items:   items,
+		Context:       formatContextBlock(header, lines),
+		Items:         items,
+		RetrievalMode: mode,
 	}, nil
 }