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/retrieval.go

@@ -11,10 +11,16 @@ import (
 	thoughttypes "git.warky.dev/wdevs/amcs/internal/types"
 )
 
+const (
+	RetrievalModeSemantic = "semantic"
+	RetrievalModeText     = "text"
+)
+
 // semanticSearch runs vector similarity search if embeddings exist for the
 // primary embedding model in the given scope, otherwise falls back to Postgres
 // full-text search. Search always uses the primary model so query vectors
 // match rows stored under the primary model name.
+// It returns the results and the retrieval mode used ("semantic" or "text").
 func semanticSearch(
 	ctx context.Context,
 	db *store.DB,
@@ -25,20 +31,22 @@ func semanticSearch(
 	threshold float64,
 	projectID *int64,
 	excludeID *uuid.UUID,
-) ([]thoughttypes.SearchResult, error) {
+) ([]thoughttypes.SearchResult, string, error) {
 	model := embeddings.PrimaryModel()
 	hasEmbeddings, err := db.HasEmbeddingsForModel(ctx, model, projectID)
 	if err != nil {
-		return nil, err
+		return nil, "", err
 	}
 
 	if hasEmbeddings {
 		embedding, err := embeddings.EmbedPrimary(ctx, query)
 		if err != nil {
-			return nil, err
+			return nil, "", err
 		}
-		return db.SearchSimilarThoughts(ctx, embedding, model, threshold, limit, projectID, excludeID)
+		results, err := db.SearchSimilarThoughts(ctx, embedding, model, threshold, limit, projectID, excludeID)
+		return results, RetrievalModeSemantic, err
 	}
 
-	return db.SearchThoughtsText(ctx, query, limit, projectID, excludeID)
+	results, err := db.SearchThoughtsText(ctx, query, limit, projectID, excludeID)
+	return results, RetrievalModeText, err
 }