fix: remove /prompt slash command — footgun via prefix expansion (#6752)

/pr <anything> silently resolved to /prompt via the shortest-match
tiebreaker in prefix expansion, permanently overwriting the system
prompt and persisting to config. The command's functionality (setting
agent.system_prompt) is available via config.yaml and /personality
covers the common use case.

Removes: CommandDef, dispatch branch, _handle_prompt_command handler,
docs references, and updates subcommand extraction test.

hermes_cli/commands.py

@@ -87,8 +87,7 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("model", "Switch model for this session", "Configuration", args_hint="[model] [--global]"),
     CommandDef("provider", "Show available providers and current provider",
                "Configuration"),
-    CommandDef("prompt", "View/set custom system prompt", "Configuration",
-               cli_only=True, args_hint="[text]", subcommands=("clear",)),
+
     CommandDef("personality", "Set a predefined personality", "Configuration",
                args_hint="[name]"),
     CommandDef("statusbar", "Toggle the context/model status bar", "Configuration",