fix: use OSC52 clipboard for remote macOS TUI

[HamsteRider-m]

Summary

• keep the existing macOS pbcopy path for local Terminal.app sessions
• fall back to Textual/OSC52 when the TUI appears to be running through SSH or tmux
• add a GA_TUI_CLIPBOARD=osc52 escape hatch for users who need to force terminal clipboard output

Why

frontends/tuiapp_v2.py currently returns immediately after pbcopy succeeds on macOS. When the TUI runs on a remote Mac over SSH/tmux, that copies to the remote Mac clipboard instead of the user's visible terminal clipboard. Allowing OSC52 in remote/tmux sessions lets compatible terminals/tmux propagate the copy to the local clipboard.

Refs #351.

Testing

• PYTHONPYCACHEPREFIX=/tmp/ga-pr-pycache python -m py_compile frontends/tuiapp_v2.py
• python frontends/tuiapp_v2.py --help
• git diff --check
• monkeypatch smoke test for local macOS, SSH, tmux, and GA_TUI_CLIPBOARD=osc52 paths

[shenhao-stu]

这个问题已经被 #418 修过了，commit 54f40d9 fix(tui): use pbcopy on macOS for clipboard (OSC 52 broken on Terminal.app)。

当前实现：frontends/tuiapp_v2.py 的 copy_to_clipboard() —— macOS 走 pbcopy，其他平台 OSC52 fallback，与你提的方向一致。先关掉，感谢～