feat(tui): retire the tui2 experiment

[joshka-oai]

Summary

• Retire the experimental TUI2 implementation and its feature flag.
• Remove TUI2-only config/schema/docs so the CLI stays on the terminal-native path.
• Keep docs aligned with the legacy TUI while we focus on redraw-based improvements.

Customer impact

• Retires the TUI2 experiment and keeps Codex on the proven terminal-native UI while we invest in redraw-based improvements to the existing experience.

Migration / compatibility

• If you previously set tui2-related options in config.toml, they are now ignored and Codex continues using the existing terminal-native TUI (no action required).

Context

• What worked: a transcript-owned viewport delivered excellent resize rewrap and high-fidelity copy (especially for code).
• Why stop: making that experience feel fully native across the environment matrix (terminal emulator, OS, input modality, multiplexer, font/theme, alt-screen behavior) creates a combinatorial explosion of edge cases.
• What next: we are focusing on redraw-based improvements to the existing terminal-native TUI so scrolling, selection, and copy remain native while resize/redraw correctness improves.

Testing

• just write-config-schema
• just fmt
• cargo clippy --fix --all-features --tests --allow-dirty --allow-no-vcs -p codex-core
• cargo clippy --fix --all-features --tests --allow-dirty --allow-no-vcs -p codex-cli
• cargo check
• cargo test -p codex-core
• cargo test -p codex-cli