feat: add /yolo slash command to toggle dangerous command approvals (#3990)

Adds a /yolo command that toggles HERMES_YOLO_MODE at runtime, skipping
all dangerous command approval prompts for the current session. Works in
both CLI and gateway (Telegram, Discord, etc.).

- /yolo -> ON: all commands auto-approved, no confirmation prompts
- /yolo -> OFF: normal approval flow restored

The --yolo CLI flag already existed for launch-time opt-in. This adds
the ability to toggle mid-session without restarting.

Session-scoped — resets when the process ends. Uses the existing
HERMES_YOLO_MODE env var that check_all_command_guards() already
respects.

hermes_cli/commands.py

@@ -90,6 +90,8 @@ COMMAND_REGISTRY: list[CommandDef] = [
     CommandDef("verbose", "Cycle tool progress display: off -> new -> all -> verbose",
                "Configuration", cli_only=True,
                gateway_config_gate="display.tool_progress_command"),
+    CommandDef("yolo", "Toggle YOLO mode (skip all dangerous command approvals)",
+               "Configuration"),
     CommandDef("reasoning", "Manage reasoning effort and display", "Configuration",
                args_hint="[level|show|hide]",
                subcommands=("none", "low", "minimal", "medium", "high", "xhigh", "show", "hide", "on", "off")),