TUI: native <scrollbox> scrolling, retire terminalScrollOffset (#212 P0, phase 2)

[windoliver]

Phase 2 of the #212 OpenTUI adoption plan. Design spec: docs/superpowers/specs/2026-05-30-tui-opentui-adoption-design.md.

Scope

• Replace the terminalScrollOffset reducer + TERMINAL_SCROLL_* actions (src/tui/app-reducer.ts) with native <scrollbox> keyboard nav (arrows / PageUp-Down / Home/End) and scrollTo/scrollBy.
• stickyScroll + stickyStart:"bottom" for log / terminal / chat views.
• Audit the ~18 panels for manual scroll handling and migrate them to the scrollbox API.

Acceptance

• No manual scroll-offset state remains.
• Log views auto-stick to bottom; keyboard scrolling works in focused panels.

Version: 0.1.87. Depends on phase 1 (typed <scrollbox>). Part of #212.