feat(gateway): add allowed_{chats,channels,rooms} whitelist to Telegram, Mattermost, Matrix, DingTalk

Mirrors the Slack `allowed_channels` feature (PR #7401) and Discord's
`allowed_channels` (PR #7044) across the remaining group-capable platforms.
All five platforms (Slack + Discord + the four added here) now follow the
same pattern: primary config via config.yaml, env-var fallback as an escape
hatch — matching the project policy that .env is for secrets only and
behavioral settings belong in config.yaml.

Also fixes a duplicate `slack` key in DEFAULT_CONFIG introduced by PR
#7401 (the later entry silently overwrote `allowed_channels`, `require_mention`,
and `free_response_channels` at dict-literal evaluation time).

Platforms added:
- Telegram: `telegram.allowed_chats` (env alias: `TELEGRAM_ALLOWED_CHATS`)
- Mattermost: `mattermost.allowed_channels` (env alias: `MATTERMOST_ALLOWED_CHANNELS`)
- Matrix: `matrix.allowed_rooms` (env alias: `MATRIX_ALLOWED_ROOMS`)
- DingTalk: `dingtalk.allowed_chats` (env alias: `DINGTALK_ALLOWED_CHATS`)

Mattermost and Matrix previously had NO config.yaml bridging for any of
their gating settings; this PR adds `load_gateway_config` bridges for them
(Mattermost gets require_mention + free_response_channels + allowed_channels;
Matrix gets allowed_rooms on top of its existing bridges for require_mention
and free_response_rooms).

Semantics identical everywhere:
- Empty = no restriction (fully backward compatible).
- Non-empty = hard whitelist: non-listed chats are silently ignored,
  even when the bot is @mentioned.
- DMs bypass the check entirely.

DEFAULT_CONFIG merges the duplicate `slack` block and adds new `mattermost`
and `matrix` blocks so all gating settings surface in defaults.

Not included: Feishu (has its own per-chat `chat_rules` system that covers
this use case differently), WhatsApp (already has `group_allow_from` via
`group_policy: allowlist`), pure-DM platforms (Signal, SMS, BlueBubbles,
Yuanbao — no group concept).

gateway/platforms/mattermost.py

@@ -706,10 +706,30 @@ class MattermostAdapter(BasePlatformAdapter):
         message_text = post.get("message", "")
 
         # Mention-gating for non-DM channels.
-        # Config (env vars):
-        #   MATTERMOST_REQUIRE_MENTION: Require @mention in channels (default: true)
-        #   MATTERMOST_FREE_RESPONSE_CHANNELS: Channel IDs where bot responds without mention
+        # Config (config.yaml `mattermost.*` with env-var fallback):
+        #   require_mention / MATTERMOST_REQUIRE_MENTION: Require @mention in channels (default: true)
+        #   free_response_channels / MATTERMOST_FREE_RESPONSE_CHANNELS: Channel IDs where bot responds without mention
+        #   allowed_channels / MATTERMOST_ALLOWED_CHANNELS: If set, bot ONLY responds in these channels (whitelist)
         if channel_type_raw != "D":
+            # allowed_channels check (whitelist — must pass before other gating).
+            # When set, messages from channels NOT in this list are silently
+            # ignored, even if @mentioned.  DMs are already excluded above.
+            allowed_raw = self.config.extra.get("allowed_channels") if self.config.extra else None
+            if allowed_raw is None:
+                allowed_raw = os.getenv("MATTERMOST_ALLOWED_CHANNELS", "")
+            if isinstance(allowed_raw, list):
+                allowed_channels = {str(c).strip() for c in allowed_raw if str(c).strip()}
+            else:
+                allowed_channels = {
+                    c.strip() for c in str(allowed_raw).split(",") if c.strip()
+                }
+            if allowed_channels and channel_id not in allowed_channels:
+                logger.debug(
+                    "Mattermost: ignoring message in non-allowed channel: %s",
+                    channel_id,
+                )
+                return
+
             require_mention = os.getenv(
                 "MATTERMOST_REQUIRE_MENTION", "true"
             ).lower() not in ("false", "0", "no")