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/model.go

@@ -227,7 +227,12 @@ type Model struct {
 	wfVisible     bool
 	wfPendingID   string // workflow chosen, awaiting an intent line before StartSession
 	wfPendingName string
-	bgUpdates     int
+	// launcher (idle screen): the active "what to launch" selection — 0 = chat, 1..N =
+	// workflows[i-1] — cycled with Tab and shown at the input's lower-right. railHidden
+	// folds away the idle status/quick-keys rail.
+	launcherWf int
+	railHidden bool
+	bgUpdates  int
 
 	// input
 	editMode     EditMode