feat(tui): retire the tui2 experiment (#9640)

## 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

codex-rs/tui/src/bottom_pane/AGENTS.md

@@ -6,8 +6,7 @@ When changing the paste-burst or chat-composer state machines in this folder, ke
   readable, top-down explanation of the current behavior.
 - Update the narrative doc `docs/tui-chat-composer.md` whenever behavior/assumptions change (Enter
   handling, retro-capture, flush/clear rules, `disable_paste_burst`, non-ASCII/IME handling).
-- Keep `codex-rs/tui` and `codex-rs/tui2` implementations/docstrings aligned unless the divergence
-  is intentional and documented.
+- Keep implementations/docstrings aligned unless a divergence is intentional and documented.
 
 Practical check: