Move the vim-mode `:` command dispatcher (`:list`, `:nick`, `:msg`,
`:last`, `:search`, `:mute-joins`, `:help`, `:clear`, `:q`, …) out of
ssh_server.c into a dedicated module.
New API (include/commands.h):
- commands_dispatch(client_t *) -- single entry point invoked from
handle_key when Enter is pressed in MODE_COMMAND.
ssh_server.c shrinks from 1769 to 1513 lines (-256).
Behaviour preserved: implementation is byte-for-byte the same.
