feat(cli): add display.vim_mode config and register /vim command

Introduces the opt-in surface for vi editing in the input composer:

- display.vim_mode config default (False, so existing users are
  unaffected and prompt_toolkit keeps its standard emacs bindings)
- /vim command registered with on|off|status subcommands, matching
  the established /battery and /timestamps pattern

Part of #4254.

hermes_cli/commands.py

@@ -163,6 +163,9 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("battery", "Toggle a color-coded battery indicator in the status bar",
                "Configuration", cli_only=True, args_hint="[on|off|status]",
                subcommands=("on", "off", "status")),
+    CommandDef("vim", "Toggle vim keybindings in the input composer",
+               "Configuration", cli_only=True, args_hint="[on|off|status]",
+               subcommands=("on", "off", "status")),
     CommandDef("timestamps", "Toggle [HH:MM] timestamps on messages and /history", "Configuration",
                cli_only=True, args_hint="[on|off|status]",
                subcommands=("on", "off", "status"), aliases=("ts",)),

hermes_cli/config_defaults.py

@@ -837,6 +837,9 @@ DEFAULT_CONFIG = {
         # fights terminal auto-scroll in non-fullscreen mode.
         # See #45592.
         "cli_refresh_interval": 1.0,
+        # Vi/vim keybindings in the CLI input composer (toggled by /vim).
+        # Off by default, preserving prompt_toolkit's standard emacs bindings.
+        "vim_mode": False,
         "user_message_preview": {  # CLI: submitted user-message lines echoed to scrollback
             "first_lines": 2,
             "last_lines": 2,