improve(session): use first user message as preview fallback in session list

When a session has no summary (most short conversations), show the first
user message content instead. Display priority: summary > preview > "(empty)".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/commands/cmd_session.go

@@ -81,7 +81,7 @@ func formatSessionList(list []session.SessionMeta) string {
 			activeMarker = "[*]"
 		}
 
-		summary := truncateSummary(item.Summary, 40)
+		summary := sessionLabel(item.Summary, item.Preview, 40)
 
 		age := "-"
 		if !item.UpdatedAt.IsZero() {
@@ -98,16 +98,21 @@ func formatSessionList(list []session.SessionMeta) string {
 	return strings.Join(lines, "\n")
 }
 
-func truncateSummary(s string, maxLen int) string {
-	s = strings.TrimSpace(s)
-	if s == "" {
-		return "(no summary)"
+// sessionLabel picks the best display text for a session list row:
+// summary (if available) > first user message preview > "(empty)".
+func sessionLabel(summary, preview string, maxLen int) string {
+	text := strings.TrimSpace(summary)
+	if text == "" {
+		text = strings.TrimSpace(preview)
 	}
-	s = strings.ReplaceAll(s, "\n", " ")
-	if len(s) <= maxLen {
-		return s
+	if text == "" {
+		return "(empty)"
 	}
-	return s[:maxLen] + "..."
+	text = strings.ReplaceAll(text, "\n", " ")
+	if len(text) <= maxLen {
+		return text
+	}
+	return text[:maxLen] + "..."
 }
 
 // extractSessionTag returns the "#N" suffix from a session key, or "#1" for

pkg/commands/session_handlers_test.go

@@ -145,6 +145,47 @@ func TestSessionHandlers_SessionList(t *testing.T) {
 	}
 }
 
+func TestSessionHandlers_SessionList_PreviewFallback(t *testing.T) {
+	ops := &fakeSessionOps{
+		listValue: []session.SessionMeta{
+			{
+				Ordinal:    1,
+				SessionKey: "scope#2",
+				UpdatedAt:  time.Date(2026, 3, 1, 9, 0, 0, 0, time.UTC),
+				MessageCnt: 3,
+				Active:     true,
+				Preview:    "How do I fix this login bug?",
+			},
+			{
+				Ordinal:    2,
+				SessionKey: "scope",
+				MessageCnt: 0,
+			},
+		},
+	}
+	rt := &Runtime{SessionOps: ops}
+	ex := NewExecutor(NewRegistry(BuiltinDefinitions()), rt)
+
+	var reply string
+	res := ex.Execute(context.Background(), Request{
+		ScopeKey: "scope",
+		Text:     "/session list",
+		Reply:    func(text string) error { reply = text; return nil },
+	})
+
+	if res.Outcome != OutcomeHandled {
+		t.Fatalf("outcome=%v, want=%v", res.Outcome, OutcomeHandled)
+	}
+	// Row 1: no summary → falls back to preview
+	if !strings.Contains(reply, "How do I fix this login bug?") {
+		t.Fatalf("reply missing preview fallback, got %q", reply)
+	}
+	// Row 2: no summary, no preview, no messages → "(empty)"
+	if !strings.Contains(reply, "(empty)") {
+		t.Fatalf("reply missing (empty) for blank session, got %q", reply)
+	}
+}
+
 func TestSessionHandlers_NilRuntime_Unavailable(t *testing.T) {
 	ex := NewExecutor(NewRegistry(BuiltinDefinitions()), nil)
 

pkg/session/manager.go

@@ -52,6 +52,7 @@ type SessionMeta struct {
 	MessageCnt int       `json:"message_cnt"`
 	Active     bool      `json:"active"`
 	Summary    string    `json:"summary,omitempty"`
+	Preview    string    `json:"preview,omitempty"` // first user message content
 }
 
 type SessionManager struct {
@@ -742,12 +743,22 @@ func (sm *SessionManager) buildSessionMetaListLocked(scope *scopeIndex) []Sessio
 			meta.UpdatedAt = session.Updated
 			meta.MessageCnt = len(session.Messages)
 			meta.Summary = session.Summary
+			meta.Preview = firstUserMessage(session.Messages)
 		}
 		list = append(list, meta)
 	}
 	return list
 }
 
+func firstUserMessage(msgs []providers.Message) string {
+	for _, m := range msgs {
+		if m.Role == "user" && strings.TrimSpace(m.Content) != "" {
+			return m.Content
+		}
+	}
+	return ""
+}
+
 func prependSessionUnique(ordered []string, sessionKey string) []string {
 	next := make([]string, 0, len(ordered)+1)
 	next = append(next, sessionKey)