Keep the migrations themselves for late-updaters. Drop the tests that only
exercised frozen file rewrites from 4.0.0, and keep live invariants,
privileged repairs, and the migrator.
Update existing terminal configs to encode Shift+Return as CSI-u so Codex and other TUIs can distinguish it from Return without tmux seeing it as Alt+Return. Also ensures Alt+Shift+Return CSI-u bindings remain present for tmux horizontal splits across Kitty, Ghostty, and Foot.