feat: multiline input with auto-expanding textarea

Switch from textinput to textarea bubble:
- Enter submits message
- Shift+Enter / Ctrl+J inserts newline
- Input area auto-expands from 1 to 10 lines based on content
- Line numbers hidden, prompt preserved

go.sum

@@ -13,6 +13,8 @@ cloud.google.com/go/compute/metadata v0.5.0 h1:Zr0eK8JbFv6+Wi4ilXAR8FJ3wyNdpxHKJ
 cloud.google.com/go/compute/metadata v0.5.0/go.mod h1:aHnloV2TPI38yx4s9+wAZhHykWvVCfu7hQbF+9CWoiY=
 github.com/BurntSushi/toml v0.3.1 h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=
+github.com/MakeNowJust/heredoc v1.0.0 h1:cXCdzVdstXyiTqTvfqk9SDHpKNjxuom+DOlyEeQ4pzQ=
+github.com/MakeNowJust/heredoc v1.0.0/go.mod h1:mG5amYoWBHf8vpLOuehzbGGw0EHxpZZ6lCpQ4fNJ8LE=
 github.com/VikingOwl91/mistral-go-sdk v1.2.1 h1:6OQMtOzJUFcvFUEtbX9VlglUPBn+dKOrQPnyoVKlpkA=
 github.com/VikingOwl91/mistral-go-sdk v1.2.1/go.mod h1:f4emNtHUx2zSqY3V0LBz6lNI1jE6q/zh+SEU+/hJ0i4=
 github.com/anthropics/anthropic-sdk-go v1.29.0 h1:7h1ZyRflhtxyuFkdwkVuJ1LdFAYdmizvgg0gd1uvOfI=

internal/tui/app.go

@@ -8,7 +8,8 @@ import (
 	"strings"
 
 	tea "charm.land/bubbletea/v2"
-	"charm.land/bubbles/v2/textinput"
+	"charm.land/bubbles/v2/textarea"
+	"charm.land/bubbles/v2/key"
 	"charm.land/lipgloss/v2"
 	"somegit.dev/Owlibou/gnoma/internal/engine"
 	"somegit.dev/Owlibou/gnoma/internal/message"
@@ -51,7 +52,7 @@ type Model struct {
 	streamBuf   strings.Builder
 	currentRole string
 
-	input          textinput.Model
+	input          textarea.Model
 	cwd            string
 	gitBranch      string
 	scrollOffset   int
@@ -61,11 +62,21 @@ type Model struct {
 }
 
 func New(sess session.Session, cfg Config) Model {
-	ti := textinput.New()
-	ti.Placeholder = ""
+	ti := textarea.New()
+	ti.Placeholder = "Type a message... (Enter to send, Shift+Enter for newline)"
 	ti.Prompt = "❯ "
-	ti.Focus()
+	ti.ShowLineNumbers = false
+	ti.SetHeight(1)
+	ti.MaxHeight = 10
 	ti.SetWidth(80)
+	ti.CharLimit = 0
+
+	// Remap: Shift+Enter/Ctrl+J for newline (not plain Enter)
+	km := ti.KeyMap
+	km.InsertNewline = key.NewBinding(key.WithKeys("shift+enter", "ctrl+j"))
+	ti.KeyMap = km
+
+	ti.Focus()
 
 	cwd, _ := os.Getwd()
 	gitBranch := detectGitBranch()
@@ -451,6 +462,16 @@ func (m Model) View() tea.View {
 		return tea.NewView("")
 	}
 
+	// Auto-size textarea based on content
+	lines := strings.Count(m.input.Value(), "\n") + 1
+	if lines < 1 {
+		lines = 1
+	}
+	if lines > 10 {
+		lines = 10
+	}
+	m.input.SetHeight(lines)
+
 	status := m.renderStatus()
 	input := m.renderInput()
 	topLine, bottomLine := m.renderSeparators()