docs(client): document gl/gL list change-symbol mappings

Add gl<sym>/gL<sym> to the lists keymap tables in README and help, noting
that 1) is command-only. Tick parity gap P1 #2.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -482,6 +482,8 @@ via `mappings.<group> = false` in Neovim, or
 | `gLh` / `gLl` | Dedent / indent including the item's sublist |
 | `glr` | Renumber the current ordered list |
 | `gLr` | Renumber every ordered list in the buffer |
+| `gl<sym>` | Set the current item's marker to `<sym>` — one of `-` `*` `#` `1` (`1.`) `i` (`i)`) `I` (`I)`) `a` (`a)`) `A` (`A)`) |
+| `gL<sym>` | Set every item in the list to `<sym>` (same symbol keys) |
 | `gl<Space>` | Remove every completed item from the current list |
 | `gL<Space>` | Remove every completed item in the buffer |
 | `o` / `O` | Open a new line and continue the list marker (preserves any checkbox) |