New HelpView modal lists all slash commands with descriptions and all
keybindings, with a live substring filter. Bound to `?` when focus is
outside the composer; Esc / `?` toggles.
Slash commands pull from the existing slash_menu registry; keybindings
pull from a new KeybindingCatalog single-source-of-truth so docs can't
drift from the wired handlers.
Hunter Bown
b759e3f74c