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/links.go

@@ -45,7 +45,8 @@ type RelatedThought struct {
 }
 
 type RelatedOutput struct {
-	Related []RelatedThought `json:"related"`
+	Related       []RelatedThought `json:"related"`
+	RetrievalMode string           `json:"retrieval_mode,omitempty"`
 }
 
 func NewLinksTool(db *store.DB, embeddings *ai.EmbeddingRunner, search config.SearchConfig) *LinksTool {
@@ -117,11 +118,13 @@ func (t *LinksTool) Related(ctx context.Context, _ *mcp.CallToolRequest, in Rela
 		includeSemantic = *in.IncludeSemantic
 	}
 
+	var retrievalMode string
 	if includeSemantic {
-		semantic, err := semanticSearch(ctx, t.store, t.embeddings, t.search, thought.Content, t.search.DefaultLimit, t.search.DefaultThreshold, thought.ProjectID, &thought.GUID)
+		semantic, mode, err := semanticSearch(ctx, t.store, t.embeddings, t.search, thought.Content, t.search.DefaultLimit, t.search.DefaultThreshold, thought.ProjectID, &thought.GUID)
 		if err != nil {
 			return nil, RelatedOutput{}, err
 		}
+		retrievalMode = mode
 		for _, item := range semantic {
 			key := fmt.Sprint(item.ID)
 			if _, ok := seen[key]; ok {
@@ -138,5 +141,5 @@ func (t *LinksTool) Related(ctx context.Context, _ *mcp.CallToolRequest, in Rela
 		}
 	}
 
-	return nil, RelatedOutput{Related: related}, nil
+	return nil, RelatedOutput{Related: related, RetrievalMode: retrievalMode}, nil
 }