feat(tui-go): idle launcher — centered input + status/keys rail (Phase 1)

Replace the idle two-panel layout (session list + welcome, where the welcome
panel duplicated the list) with an opencode-style launcher: a compact, centered
input whose lower-right shows the launch target and model (chat by default), a
hideable right rail of status + quick keys, and no session list — it's a keypress
away via R.

- Tab (or w) cycles the launch target: chat → workflows → chat (launcherWf),
  shown at the input. Enter on chat starts a chat; on a workflow, StartSession
  with the typed text as the brief.
- Right rail (connection, session/active counts, i/Tab/R/?/p keys) hides via the
  new "rail" palette command. Drops automatically on narrow terminals.
- View() suppresses the bottom input bar on idle (the input is the launcher);
  footer + ? help updated to the launcher keys (help-coverage test extended).

The old idle-panel renderers (sessionRows/welcomeRows/workflowRows) are now
unused but kept in the tree pending Phase 2 + a decision on where the session
dates should live now that the idle list is gone. Multi-line input (Ctrl+J /
Alt+Enter newline) is Phase 2.

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

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

@@ -821,6 +821,7 @@ func (m Model) helpBody() []string {
 	section("navigate", []kb{
 		{"↑↓ / j k", "move the session / list selection"},
 		{"enter", "open the selected session"},
+		{"Tab / w", "idle: cycle launch target (chat / workflow)"},
 		{"/", "filter the session list"},
 		{"^↑ / ^↓", "jump to your previous / next message"},
 		{"PgUp / PgDn", "scroll output (^u / ^d half-page)"},