Typing `/` as the first char of a chat line opens the command palette inline
(opencode-style), instead of typing a literal slash; mid-line `/` stays literal.
Factors openPalette() (shared with the `p` key) and advertises `/ cmds` in the
insert-mode footer when the line is empty.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
