feat(tui): support configurable tab positions

[kitlangton]

What

Allow V2 TUI tabs to be positioned at the top, bottom, left, or right through the Tabs settings surface.

Horizontal positions retain the one-line tab strip. Side positions retain the existing 42-column vertical sidebar and fall back to the top strip when the terminal is too narrow.

How

• Replace the beta tabs.layout setting with one tabs.position enum: top, bottom, left, or right.
• Compose the existing horizontal and vertical tab renderers at the selected edge in packages/tui/src/app.tsx.
• Share effective side-position and reserved-width calculations with the session route so transcript and sidebar sizing remain aligned.
• Default to top; this beta setting intentionally does not migrate the previous layout value.

Scope

This changes only the V2 TUI and its client-local setting. It does not change tab persistence, tab renderer behavior, or the V1 implementation.

Testing

• cd packages/tui && bun run test (611 passed, 5 skipped)
• cd packages/tui && bun run test test/config-v2.test.tsx test/ui/layout.test.ts (8 passed)
• cd packages/tui && bun typecheck
• cd packages/cli && bun run test test/config.test.ts (4 passed)
• cd packages/cli && bun typecheck
• Push hook: workspace bun turbo typecheck --concurrency=3 (33 packages passed)
• OpenCode Drive scripted run at 120x36 switched through the settings UI and asserted the visible geometry for all four positions; four screenshots and a recording were produced.

Demo

The recording shows the Tabs Position setting switching from top to bottom to left to right, with each resulting layout visible before the next switch.

recording-8f8ae030-b4e4-4815-a7c5-30cdfabb45d8.mp4