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_test.go

@@ -0,0 +1,60 @@
+package tools
+
+import "testing"
+
+func TestRetrievalModeConstants(t *testing.T) {
+	if RetrievalModeSemantic != "semantic" {
+		t.Fatalf("RetrievalModeSemantic = %q, want %q", RetrievalModeSemantic, "semantic")
+	}
+	if RetrievalModeText != "text" {
+		t.Fatalf("RetrievalModeText = %q, want %q", RetrievalModeText, "text")
+	}
+}
+
+func TestSearchOutputIncludesRetrievalMode(t *testing.T) {
+	out := SearchOutput{RetrievalMode: RetrievalModeSemantic}
+	if out.RetrievalMode != RetrievalModeSemantic {
+		t.Fatalf("SearchOutput.RetrievalMode = %q, want %q", out.RetrievalMode, RetrievalModeSemantic)
+	}
+}
+
+func TestRelatedOutputIncludesRetrievalMode(t *testing.T) {
+	out := RelatedOutput{RetrievalMode: RetrievalModeText}
+	if out.RetrievalMode != RetrievalModeText {
+		t.Fatalf("RelatedOutput.RetrievalMode = %q, want %q", out.RetrievalMode, RetrievalModeText)
+	}
+	// retrieval_mode is omitempty — empty string means semantic search was skipped
+	out2 := RelatedOutput{}
+	if out2.RetrievalMode != "" {
+		t.Fatalf("RelatedOutput.RetrievalMode = %q, want empty when include_semantic is false", out2.RetrievalMode)
+	}
+}
+
+func TestSummarizeOutputIncludesRetrievalModeOnlyWhenQueryUsed(t *testing.T) {
+	withQuery := SummarizeOutput{Summary: "s", Count: 1, RetrievalMode: RetrievalModeSemantic}
+	if withQuery.RetrievalMode != RetrievalModeSemantic {
+		t.Fatalf("SummarizeOutput.RetrievalMode = %q, want %q", withQuery.RetrievalMode, RetrievalModeSemantic)
+	}
+	withoutQuery := SummarizeOutput{Summary: "s", Count: 1}
+	if withoutQuery.RetrievalMode != "" {
+		t.Fatalf("SummarizeOutput.RetrievalMode = %q, want empty when no query", withoutQuery.RetrievalMode)
+	}
+}
+
+func TestProjectContextOutputIncludesRetrievalModeOnlyWhenQueryUsed(t *testing.T) {
+	withQuery := ProjectContextOutput{RetrievalMode: RetrievalModeText}
+	if withQuery.RetrievalMode != RetrievalModeText {
+		t.Fatalf("ProjectContextOutput.RetrievalMode = %q, want %q", withQuery.RetrievalMode, RetrievalModeText)
+	}
+	withoutQuery := ProjectContextOutput{}
+	if withoutQuery.RetrievalMode != "" {
+		t.Fatalf("ProjectContextOutput.RetrievalMode = %q, want empty when no query", withoutQuery.RetrievalMode)
+	}
+}
+
+func TestRecallOutputIncludesRetrievalMode(t *testing.T) {
+	out := RecallOutput{RetrievalMode: RetrievalModeSemantic}
+	if out.RetrievalMode != RetrievalModeSemantic {
+		t.Fatalf("RecallOutput.RetrievalMode = %q, want %q", out.RetrievalMode, RetrievalModeSemantic)
+	}
+}