fix(gateway): use correct variable for skill slash command task_id

Line 1482 referenced 'session_key' which is not defined until line 1519,
causing a NameError on every skill slash command invocation in the gateway
(e.g. /deploy, /plan-with-skill). The try/except silently swallowed the
error, making all user-defined skill slash commands silently fail.

The correct variable is '_quick_key', defined at line 1292 (same variable
used by the /plan handler on line 1379).

gateway/run.py

@@ -1481,7 +1481,7 @@ class GatewayRunner:
                 if cmd_key in skill_cmds:
                     user_instruction = event.get_command_args().strip()
                     msg = build_skill_invocation_message(
-                        cmd_key, user_instruction, task_id=session_key
+                        cmd_key, user_instruction, task_id=_quick_key
                     )
                     if msg:
                         event.text = msg