Move the conversation stream to append-only; drop the self-managed virtual-view layer

Background

App.tsx currently re-renders the entire conversation history as a dynamic Ink tree. To support that, we maintain a large pile of "we manage the screen ourselves" machinery:

• A byte-level virtual frame layer (src/frame/* — viewport / slice / overlay / bottom)
• Viewport budgeting (layout/viewport-budget.tsx, prompt-viewport.ts)
• Chat scroll state + scrollback hook (state/chat-scroll-store.ts, hooks/useScrollback.ts)
• An application-level copy-mode (copy-mode/* — press q to rebuild a selection region)

The price is that terminal behavior is inconsistent across emulators. Scrolling, copy, resize, line wrapping — each one fights the terminal for the same pixels, and each terminal behaves differently. Directly related issues: #1527 (mouse capture breaks native selection), #1523 (cursor disappears on Ghostty).

Direction

Adopt the Claude Code model:

• History region → Ink <Static>. Rendered once, written through process.stdout.write, then Ink stops touching it. The terminal owns native scrollback / copy / mouse wheel.
• Live region → A small zone at the bottom (composer, spinner, status line). Dynamic redraws only patch the last few lines.
• Full-screen scenes (Setup / CheckpointPicker / McpHub) → Enter alt-screen for the duration, exit back to the append-only main stream. They never compete with history for screen space.
• Streaming messages → Render progressively in the live region; commit to <Static> only on finalize.

Stages

In dependency order, each stage is its own PR:

• Stage 1 — <Static> parallel validation: Move the history region in App.tsx to <Static>. Keep the old path behind a flag and run them in parallel until every card type finalizes correctly.
• Stage 2 — Drop copy-mode / chat-scroll-store / useScrollback: Once the terminal owns scrolling and selection, these have no callers. Grep for dead refs, then delete.
• Stage 3 — Drop viewport-budget / prompt-viewport: After confirming the composer no longer depends on them.
• Stage 4 — Drop src/frame/*: The lowest layer, removed last. If anything in frame/ansi.ts or frame/width.ts is still useful, lift it into cli/ui/text-utils.ts.

Net deletion target: ~2.5–3k lines.

Tradeoff we're accepting

The application-level copy-mode goes away. Copy relies on the terminal: native mouse selection, Ctrl+Shift+C, tmux copy-mode. Every modern emulator handles this. The small population on pure ssh + mouseless tmux loses application-level selection — accepted. As a bonus, #1527 resolves automatically: we no longer need to capture the mouse outside alt-screen scenes.

Out of scope

• Implementation details of putting Setup / CheckpointPicker / McpHub into alt-screen (separate issue)
• Incremental syntax highlighting for streaming markdown in the live region (separate issue)