Add optional intro message on bot startup

When SEND_INTRO_MESSAGE=true, the bot sends "The deals must flow." as
an m.notice to the Matrix room right after startup, before the first
scheduled deal checks. Defaults to off.

https://claude.ai/code/session_017eMsVwUopgmnEyd6JJedpV

gaming_deals_bot/bot.py

@@ -72,6 +72,12 @@ class DealsBot:
         total = len(deals) + len(current_free) + len(upcoming)
         logger.info("First run: recorded %d existing deals/games", total)
 
+    async def send_intro(self):
+        """Send an intro message to the Matrix room if configured."""
+        if not self.config.send_intro_message:
+            return
+        await self.matrix.send_notice("The deals must flow.")
+
     async def check_cheapshark(self):
         """Poll CheapShark for deals and post new ones."""
         if not self._first_run_done: