feat: add configurable Space leader bindings

src/main.rs

@@ -8,6 +8,8 @@ Usage: vamp [--config PATH | --no-config] [FILE]
 
 Default keys (configurable with config.bind):
 All modes: Ctrl+p command palette; type to filter, Up/Down select, Enter accept, Esc cancel
+Normal/Visual: Space is leader; Space Space opens palette; Esc cancels prefix
+Normal leader: Space w saves; Space b n/p/l/d next/previous/list/close buffer
 Normal: i/a insert, I/A insert at first nonblank/end, o/O new line
         h/j/k/l move, 0/$ line ends, x delete, J join lines
         w/b/e word next/previous/end, W/B/E whitespace-delimited WORDs