feat(tui-go): multi-line input — Ctrl+J / Alt+Enter newline, growing box (Phase 2)

Enter still submits; Ctrl+J (always) and Alt+Enter (most terminals) insert a
newline at the cursor. True Shift+Enter isn't distinguishable from Enter in
bubbletea v1.3.10 (terminals send the same byte, no kitty-protocol parsing), so
these are the working stand-ins — noted in the input hint.

- editorLines() renders the buffer as styled rows with the caret at the cursor,
  splitting on \n and capping at maxInputLines (last rows kept). Both the idle
  launcher input and the in-session bottom bar use it and grow to fit; View()
  sizes the bottom bar via inputBarHeight() instead of the fixed inputH.
- Footer gains a "^J newline" hint in insert mode (non-filter); the launcher
  sub-line shows "⌥↵/^J newline". New "compose" preview kind + editor_lines_test
  (split + height cap).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/tui-go/internal/app/view.go

@@ -38,7 +38,7 @@ func (m Model) View() string {
 	}
 
 	ds := m.displayState()
-	bottom, bottomH := m.renderInput(), inputH
+	bottom, bottomH := m.renderInput(), m.inputBarHeight()
 	switch {
 	case ds == StateApproval:
 		bottomH = m.approvalBandHeight()
@@ -230,6 +230,9 @@ func (m Model) renderFooter() string {
 	switch {
 	case m.editMode == ModeInsert:
 		hints = []string{hint("enter", "send"), hint("esc", "normal"), hint("↑↓", "history")}
+		if m.inputMode != ModeFilter {
+			hints = append(hints, hint("^J", "newline"))
+		}
 		// The `/` command menu only triggers on an empty chat line (mid-line `/` is literal).
 		if m.inputMode == ModeRouter && m.inputBuffer == "" {
 			hints = append(hints, hint("/", "cmds"))
@@ -326,30 +329,67 @@ func (m Model) renderLauncher(w, h int) string {
 	return lipgloss.JoinHorizontal(lipgloss.Top, left, rail)
 }
 
-// launcherInput renders the compact input box plus a status sub-line (<workflow> · <model>
-// left, action hints right).
+// maxInputLines caps how many lines a growing (multi-line) input box shows; beyond it the
+// box scrolls to keep the cursor's tail in view.
+const maxInputLines = 6
+
+// editorLines renders the input buffer as styled display lines with the caret at the cursor,
+// so multi-line input (Ctrl+J / Alt+Enter) shows across rows. Tabs are flattened to a space.
+func (m Model) editorLines() []string {
+	t := m.theme
+	caret := ""
+	if m.editMode == ModeInsert && m.caretVisible() {
+		caret = lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.Bg).Render("▏")
+	}
+	buf := m.inputBuffer
+	cur := m.inputCursor
+	if cur > len(buf) {
+		cur = len(buf)
+	}
+	if cur < 0 {
+		cur = 0
+	}
+	style := func(s string) string { return t.span(strings.ReplaceAll(s, "\t", " "), t.P.FgStrong) }
+	before := strings.Split(buf[:cur], "\n")
+	after := strings.Split(buf[cur:], "\n")
+	var lines []string
+	for _, ln := range before[:len(before)-1] {
+		lines = append(lines, style(ln))
+	}
+	lines = append(lines, style(before[len(before)-1])+caret+style(after[0]))
+	for _, ln := range after[1:] {
+		lines = append(lines, style(ln))
+	}
+	if len(lines) > maxInputLines {
+		lines = lines[len(lines)-maxInputLines:]
+	}
+	return lines
+}
+
+// launcherInput renders the compact (growing) input box plus a status sub-line
+// (<workflow> · <model> left, action hints right).
 func (m Model) launcherInput(w int) string {
 	t := m.theme
 	insert := m.editMode == ModeInsert
 	prompt := lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.Bg).Bold(true).Render("› ")
-	caret := t.span(" ", t.P.Bg)
-	if insert && m.caretVisible() {
-		caret = lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.Bg).Render("▏")
+	indent := t.span("  ", t.P.Bg)
+	var content []string
+	if m.inputBuffer == "" && !insert {
+		content = []string{prompt + t.span("Ask anything…", t.P.Faint)}
+	} else {
+		for i, ln := range m.editorLines() {
+			if i == 0 {
+				content = append(content, prompt+ln)
+			} else {
+				content = append(content, indent+ln)
+			}
+		}
 	}
-	var line string
-	switch {
-	case m.inputBuffer == "" && !insert:
-		line = prompt + t.span("Ask anything…", t.P.Faint)
-	case m.inputBuffer == "":
-		line = prompt + caret + t.span("Ask anything…", t.P.Faint)
-	default:
-		line = prompt + t.span(flattenForDisplay(m.inputBuffer), t.P.FgStrong) + caret
-	}
-	box := t.box("", []string{line}, w, 3, insert)
+	box := t.box("", content, w, len(content)+2, insert)
 
 	wf := lipgloss.NewStyle().Foreground(t.P.Accent2).Background(t.P.Bg).Render(m.launcherWfName())
 	leftSub := "  " + wf + t.span(" · ", t.P.Faint) + t.span(m.currentModel, t.P.Faint)
-	hints := t.span("Tab workflow · enter ↵  ", t.P.Faint)
+	hints := t.span("Tab wf · ⌥↵/^J newline · enter ↵  ", t.P.Faint)
 	return box + "\n" + m.justify(leftSub, hints, w, t.P.Bg)
 }
 
@@ -449,14 +489,14 @@ func (m Model) renderInput() string {
 	if insert && m.caretVisible() {
 		caret = lipgloss.NewStyle().Foreground(t.P.Accent).Background(t.P.Bg).Render("▏")
 	}
-	var line string
+	var content []string
 	switch {
 	case m.inputBuffer == "" && !insert:
-		line = t.span(placeholder, t.P.Faint)
+		content = []string{t.span(placeholder, t.P.Faint)}
 	case m.inputBuffer == "":
-		line = caret + t.span(placeholder, t.P.Faint)
+		content = []string{caret + t.span(placeholder, t.P.Faint)}
 	default:
-		line = t.span(flattenForDisplay(m.inputBuffer), t.P.FgStrong) + caret
+		content = m.editorLines()
 	}
 
 	// status sub-line: <session/none> · <uuid> · <mode>
@@ -482,8 +522,18 @@ func (m Model) renderInput() string {
 	sub += t.span(" · ", t.P.Faint) +
 		t.span(mode, t.P.Dim)
 
-	body := []string{line, sub}
-	return t.box("", body, m.width, inputH, insert)
+	body := append(content, sub)
+	return t.box("", body, m.width, len(body)+2, insert)
+}
+
+// inputBarHeight is the bottom input bar's current height (it grows with multi-line input):
+// content lines (capped at maxInputLines) + the status sub-line + 2 borders.
+func (m Model) inputBarHeight() int {
+	n := 1
+	if m.inputBuffer != "" {
+		n = len(m.editorLines())
+	}
+	return n + 1 + 2
 }
 
 // --- body row builders ---