added j / k for vim-style line movement

j and k move the caret one line, delegated to sublime's own `move` command
so they behave exactly like the arrow keys. They dispatch through the
existing ACTIONS registry, so the keymap, the status-bar hints and the ?
popup all stay in sync through the single KEY_GROUPS table.

Unlike the other letter keys, j / k keep no command_mode context: the view
is read-only, so leaving them live while Vintage is in command mode can
only ever move the caret. That asymmetry was previously documented only in
a comment, which was wrong (it claimed j / k were the only unguarded
bindings, but escape has always carried its own context instead). It is now
pinned by test_command_mode_guard_is_deliberate, so normalising the keymap
fails the suite with the reason attached.

Also tidies two long lines in commands.py.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -22,6 +22,7 @@ Keys inside the review view:
 | `o` or Enter | open the real file at this line |
 | `]` / `[` | next / previous change |
 | `n` / `p` | next / previous file of the current list |
+| `j` / `k` | move down / up one line (vim keys) |
 | `r` | refresh |
 | `?` | show this key list in a popup |
 | Esc | close the view |