tui: replace debug/test-only crate features with debug_assertions

## Why

The remaining `vt100-tests` and `debug-logs` features in `codex-tui` were only selecting debug and test-only behavior. Those are exactly the cases where this repo already accepts `#[cfg(debug_assertions)]` as the Cargo-free substitute.

## What changed

- delete `vt100-tests` and `debug-logs` from `codex-tui`
- gate the VT100 integration tests with `debug_assertions` instead of a Cargo feature
- gate the textarea debug log site with `debug_assertions`
- shrink the manifest verifier allowlist again so only the remaining real feature exceptions stay permitted

.github/scripts/verify_cargo_workspace_manifests.py

@@ -30,8 +30,6 @@ MANIFEST_FEATURE_EXCEPTIONS = {
     },
     "codex-rs/tui/Cargo.toml": {
         "default": ("voice-input",),
-        "vt100-tests": (),
-        "debug-logs": (),
         "voice-input": ("dep:cpal",),
     },
 }