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.

gateway/run.py

@@ -1877,6 +1877,9 @@ class GatewayRunner:
         if canonical == "verbose":
             return await self._handle_verbose_command(event)
 
+        if canonical == "yolo":
+            return await self._handle_yolo_command(event)
+
         if canonical == "provider":
             return await self._handle_provider_command(event)
         
@@ -4109,6 +4112,16 @@ class GatewayRunner:
         else:
             return f"🧠 ✓ Reasoning effort set to `{effort}` (this session only)"
 
+    async def _handle_yolo_command(self, event: MessageEvent) -> str:
+        """Handle /yolo — toggle dangerous command approval bypass."""
+        current = bool(os.environ.get("HERMES_YOLO_MODE"))
+        if current:
+            os.environ.pop("HERMES_YOLO_MODE", None)
+            return "⚠️ YOLO mode **OFF** — dangerous commands will require approval."
+        else:
+            os.environ["HERMES_YOLO_MODE"] = "1"
+            return "⚡ YOLO mode **ON** — all commands auto-approved. Use with caution."
+
     async def _handle_verbose_command(self, event: MessageEvent) -> str:
         """Handle /verbose command — cycle tool progress display mode.