Fix Shift+Return terminal encoding

Encode Shift+Return as CSI-u across supported terminals so TUIs such as Codex can distinguish it from plain Return. This replaces Alacritty's ambiguous ESC+Return mapping, which tmux interpreted as Alt+Return and used to split panes, while preserving Alt+Shift+Return as CSI-u for tmux's M-S-Enter binding.

config/alacritty/alacritty.toml

@@ -21,7 +21,8 @@ decorations = "None"
 bindings = [
 { key = "Insert", mods = "Shift", action = "Paste" },
 { key = "Insert", mods = "Control", action = "Copy" },
-{ key = "Return", mods = "Shift", chars = "\u001B\r" },
+# Send Shift+Return as CSI-u so TUIs can distinguish it from Return without treating it as Alt+Return.
+{ key = "Return", mods = "Shift", chars = "\u001B[13;2u" },
 # Legacy encoding sends Alt+Shift+Return the same as Alt+Return; send CSI-u so tmux can match M-S-Enter.
 { key = "Return", mods = "Alt|Shift", chars = "\u001B[13;4u" }
 ]