Make Alt+Enter the primary multiline input shortcut

[alecuba16]

Problem

Shift+Enter is documented as the shortcut for inserting a newline in the input composer, but it does not work in some terminals. Instead of adding a line break, the prompt is submitted as if plain Enter was pressed.

Why it happens

Some terminals do not send a distinct key event for Shift+Enter. They report it the same way as plain Enter, so the TUI cannot tell whether the user wanted to submit or insert a newline.

What could be better

Keep the current keyboard protocol behavior to avoid risking printable text and non-US keyboard layout regressions from REPORT_ALL_KEYS_AS_ESCAPE_CODES.

Make Alt+Enter the first-class documented newline shortcut because it is more compatible across terminals and is reliably reported by common terminal emulators. Keep Shift+Enter documented only as a supported-terminal shortcut.

The input hint shown while composing during a running turn should mention this directly next to the existing Ctrl/Cmd+Enter queue/send hint.

Example implementation

Compare link:
master...alecuba16:jcode:multiline-input

[1jehuang]

Triage: agreed on the underlying problem. Legacy terminals without the kitty keyboard protocol report Shift+Enter identically to Enter, so no TUI-side fix can distinguish them, and Alt+Enter is reliably distinguishable almost everywhere. Master already binds and tests Alt+Enter (new_terminal_alt_enter_binding_parses_and_matches) and the help overlay shows "Shift+Enter / Alt+Enter".

What remains is the documentation/hint-priority decision you propose: making Alt+Enter the first-listed shortcut and mentioning it in the composing hint next to Ctrl/Cmd+Enter. Marked needs-decision for the doc ordering, but the hint improvement itself looks uncontroversial. Will look at your compare branch.