Limit iTerm2 TUI capture to visible viewport

[cbusillo]

Summary

• Limit the test-tui iTerm2 text helper to the visible viewport instead of scrollback-backed screen contents.
• Include viewport range metadata in helper output so agents can tell what was captured.

Validation

• python3 -m py_compile .codex/skills/test-tui/scripts/iterm2_tui_visibility.py
• uv run /Users/cbusillo/Developer/codex-skills/skill-creator/scripts/quick_validate.py .codex/skills/test-tui
• npx prettier --check .codex/skills/test-tui/SKILL.md
• git diff --check
• ./build-fast.sh