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/view.go

@@ -221,6 +221,10 @@ func (m Model) renderFooter() string {
 	switch {
 	case m.editMode == ModeInsert:
 		hints = []string{hint("enter", "send"), hint("esc", "normal"), hint("↑↓", "history")}
+		// 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"))
+		}
 	case m.displayState() == StateApproval:
 		// Approval actions live in the band itself; the footer offers navigation only.
 		hints = []string{hint("l", "back"), hint("e", "events"), hint("q", "quit")}