test: add terminal transcript smoke coverage

[benvinegar]

Summary

• add a real terminal transcript smoke test using script and timeout
• cover split mode, stack mode, and pager mode terminal output
• add a test:tty-smoke script for targeted reruns

Why

Most rendering assertions today use test-render snapshots. Those are fast and useful, but they do not exercise the actual terminal session path. This branch adds smoke coverage that captures a real terminal transcript and checks for the kinds of regressions that are easy to miss in component-only tests.

Covered scenarios

• split mode chrome, active rails, and AI badge output
• stack mode row layout without split separators
• pager mode hiding chrome while still rendering diff content

Validation

• bun run typecheck
• bun test