feat(tui-go): slash command menu in the chat input

Typing `/` as the first char of a chat line opens the command palette inline
(opencode-style), instead of typing a literal slash; mid-line `/` stays literal.
Factors openPalette() (shared with the `p` key) and advertises `/ cmds` in the
insert-mode footer when the line is empty.

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

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

@@ -0,0 +1,45 @@
+package app
+
+import (
+	"testing"
+
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+// `/` as the first char of a chat line opens the command palette inline (opencode-style),
+// without typing a literal slash.
+func TestSlashOpensPaletteOnEmptyLine(t *testing.T) {
+	m := NewModel(nil)
+	m.editMode = ModeInsert
+	m.inputMode = ModeRouter
+	m.inputBuffer = ""
+
+	next, _ := m.handleInsertKey(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("/")})
+	nm := next.(Model)
+
+	if nm.overlay != OverlayPalette {
+		t.Fatalf("'/' on an empty line should open the palette; overlay=%d", nm.overlay)
+	}
+	if nm.inputBuffer != "" {
+		t.Fatalf("'/' should not be typed literally; buffer=%q", nm.inputBuffer)
+	}
+}
+
+// Mid-line `/` is a literal slash (e.g. a path), not a command trigger.
+func TestSlashLiteralMidLine(t *testing.T) {
+	m := NewModel(nil)
+	m.editMode = ModeInsert
+	m.inputMode = ModeRouter
+	m.inputBuffer = "path"
+	m.inputCursor = len("path")
+
+	next, _ := m.handleInsertKey(tea.KeyMsg{Type: tea.KeyRunes, Runes: []rune("/")})
+	nm := next.(Model)
+
+	if nm.overlay != OverlayNone {
+		t.Fatalf("mid-line '/' must stay literal, not open the palette; overlay=%d", nm.overlay)
+	}
+	if nm.inputBuffer != "path/" {
+		t.Fatalf("mid-line '/' should be typed; buffer=%q", nm.inputBuffer)
+	}
+}