Why

The TUI startup test surface had drifted into expensive, brittle coverage:

• tui/tests/suite/no_panic_on_startup.rs was already ignored as flaky while still spawning a PTY to exercise malformed exec-policy rules.
• tui/tests/suite/model_availability_nux.rs used a seeded session, cursor-query spoofing, and repeated interrupts to verify a narrow resume-path invariant.
• app/tests.rs had started accumulating unrelated startup and summary coverage in one flat module even after the surrounding app code was split into feature modules.

This keeps those behaviors covered while making the tests cheaper to understand and less likely to rot. It also preserves the malformed-rules regression from #8803 without requiring a terminal orchestration test.

What changed

• Replaced the malformed rules startup PTY case with a direct exec-policy loader regression:
  rules_path_file_returns_read_dir_error
• Made the existing fresh-session-only startup tooltip behavior explicit with
  should_prepare_startup_tooltip_override,
  then added focused coverage for the resume/fork gate and the persisted NUX counter.
• Split startup and session-summary coverage out of tui/src/app/tests.rs into dedicated modules so the test layout better mirrors the current app architecture.
• Converted one single-message goal validation snapshot into semantic assertions where layout was not the behavior under test.
• Removed the two PTY-heavy suite files that the narrower tests now supersede.

Verification

• cargo test -p codex-core rules_path_file_returns_read_dir_error
• cargo test -p codex-tui startup_
• cargo test -p codex-tui session_summary_
• cargo test -p codex-tui goal_slash_command_rejects_oversized_objective