feat: stabilize list output with fixed-height preview (5 lines, 300 chars)

Each session always occupies the same number of lines, preventing
visual jitter between watch refreshes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

display.go

@@ -92,10 +92,12 @@ func DisplaySessionInfos(w io.Writer, sessions []SessionInfo, noColor bool, now
 
 		fmt.Fprintf(w, "[%s] %s%s%s%s%s\n", stateStr, s.Cwd, branch, workspace, label, waiting)
 
-		if s.Preview != "" {
-			lines := strings.Split(s.Preview, "\n")
-			for _, line := range lines {
-				fmt.Fprintf(w, "         %s\n", line)
+		lines := strings.Split(s.Preview, "\n")
+		for i := 0; i < previewLines; i++ {
+			if i < len(lines) && lines[i] != "" {
+				fmt.Fprintf(w, "         %s\n", lines[i])
+			} else {
+				fmt.Fprintln(w)
 			}
 		}
 	}

state.go

@@ -64,7 +64,7 @@ func DetectState(messages []JSONLMessage, now time.Time) SessionState {
 }
 
 // ExtractPreview finds the last assistant text content and returns the first
-// 3 lines, truncated to 200 characters.
+// 5 lines, truncated to 300 characters.
 func ExtractPreview(messages []JSONLMessage) string {
 	for i := len(messages) - 1; i >= 0; i-- {
 		msg := messages[i]
@@ -80,15 +80,18 @@ func ExtractPreview(messages []JSONLMessage) string {
 	return ""
 }
 
+const previewLines = 5
+const previewMaxChars = 300
+
 func truncatePreview(text string) string {
-	lines := strings.SplitN(text, "\n", 4)
-	if len(lines) > 3 {
-		lines = lines[:3]
+	lines := strings.SplitN(text, "\n", previewLines+1)
+	if len(lines) > previewLines {
+		lines = lines[:previewLines]
 	}
 	result := strings.Join(lines, "\n")
 
-	if len(result) > 200 {
-		result = result[:200] + "..."
+	if len(result) > previewMaxChars {
+		result = result[:previewMaxChars] + "..."
 	}
 	return result
 }

state_test.go

@@ -151,13 +151,13 @@ func TestExtractPreview(t *testing.T) {
 			Type: "assistant",
 			Message: &MessagePayload{
 				Role: "assistant",
-				Content: []ContentBlock{{Type: "text", Text: "Voici le resultat\nLigne 2\nLigne 3\nLigne 4\nLigne 5"}},
+				Content: []ContentBlock{{Type: "text", Text: "Voici le resultat\nLigne 2\nLigne 3\nLigne 4\nLigne 5\nLigne 6\nLigne 7"}},
 			},
 		},
 	}
 
 	preview := ExtractPreview(msgs)
-	want := "Voici le resultat\nLigne 2\nLigne 3"
+	want := "Voici le resultat\nLigne 2\nLigne 3\nLigne 4\nLigne 5"
 	if preview != want {
 		t.Errorf("ExtractPreview = %q, want %q", preview, want)
 	}
@@ -179,7 +179,7 @@ func TestExtractPreview_NoAssistant(t *testing.T) {
 }
 
 func TestExtractPreview_LongText(t *testing.T) {
-	// Text longer than 200 chars should be truncated
+	// Text longer than 300 chars should be truncated
 	longText := ""
 	for i := 0; i < 100; i++ {
 		longText += "abcde "
@@ -194,7 +194,7 @@ func TestExtractPreview_LongText(t *testing.T) {
 	}}
 
 	preview := ExtractPreview(msgs)
-	if len(preview) > 203 { // 200 + "..."
+	if len(preview) > 303 { // 300 + "..."
 		t.Errorf("preview too long: %d chars", len(preview))
 	}
 	if preview[len(preview)-3:] != "..." {