feat(discord): add DISCORD_ALLOW_BOTS config for bot message filtering (inspired by openclaw)

Add configurable bot message filtering via DISCORD_ALLOW_BOTS env var:

- 'none' (default): Ignore all other bot messages — matches previous
  behavior where only our own bot was filtered, but now ALL bots are
  filtered by default for cleaner channels
- 'mentions': Accept bot messages only when they @mention our bot —
  useful for bot-to-bot workflows triggered by mentions
- 'all': Accept all bot messages — for setups where bots need to
  interact freely

Previously, we only ignored our own bot's messages, allowing all other
bots through. This could cause noisy loops in channels with multiple bots.

8 new tests covering all filter modes and edge cases.

Inspired by openclaw v2026.3.7 Discord allowBots: 'mentions' config.

gateway/platforms/discord.py

@@ -120,9 +120,23 @@ class DiscordAdapter(BasePlatformAdapter):
             
             @self._client.event
             async def on_message(message: DiscordMessage):
-                # Ignore bot's own messages
+                # Always ignore our own messages
                 if message.author == self._client.user:
                     return
+                
+                # Bot message filtering (DISCORD_ALLOW_BOTS):
+                #   "none"     — ignore all other bots (default)
+                #   "mentions" — accept bot messages only when they @mention us
+                #   "all"      — accept all bot messages
+                if getattr(message.author, "bot", False):
+                    allow_bots = os.getenv("DISCORD_ALLOW_BOTS", "none").lower().strip()
+                    if allow_bots == "none":
+                        return
+                    elif allow_bots == "mentions":
+                        if not self._client.user or self._client.user not in message.mentions:
+                            return
+                    # "all" falls through to handle_message
+                
                 await self._handle_message(message)
             
             # Register slash commands