feat(cli): add an in-app transcript viewer for long TUI sessions

Problem

The TUI currently renders the full transcript into the terminal buffer and relies on native terminal scrollback. That works for ordinary sessions, but after a long session (for example, around 100 turns) the earliest turns may no longer be reachable when the terminal scrollback limit is smaller than the rendered transcript.

Desktop transcript virtualization in #2975 addresses React/Electron rendering cost. It does not change the CLI TUI, which has a separate renderer and interaction model.

Desired outcome

Provide a read-only in-app transcript viewer for the TUI that:

• opens on the latest part of the current transcript;
• scrolls one line with Up/Down;
• scrolls one page with Page Up/Page Down;
• jumps to the beginning or end with Home/End;
• closes with q or Escape;
• preserves the existing double-Escape rewind flow for editing an earlier turn.

The viewer should consume the existing CLI transcript projection. It should not introduce another session-history or Runtime authority.

Alternatives or workarounds

Today users can increase terminal scrollback or use terminal/trackpad scrolling. That is terminal-dependent and cannot recover lines already discarded by the terminal.

A full-screen alternate-buffer redesign would solve a broader problem, but is a much larger behavioral change. A focused viewer behind a slash command is a smaller first step and avoids stealing editor keybindings during normal input.

Scope

This is a user-visible CLI feature. While the repository is feature-frozen, implementation PRs should remain Draft until maintainers decide whether and when to take it.