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/matrix.py

@@ -17,7 +17,8 @@ Environment variables:
     MATRIX_REACTIONS        Set "false" to disable processing lifecycle reactions
                             (eyes/checkmark/cross). Default: true
     MATRIX_REQUIRE_MENTION      Require @mention in rooms (default: true)
-    MATRIX_FREE_RESPONSE_ROOMS  Comma-separated room IDs exempt from mention requirement
+    MATRIX_FREE_RESPONSE_ROOMS  Comma-separated room IDs exempt from mention requirement (alias of matrix.free_response_rooms)
+    MATRIX_ALLOWED_ROOMS    Comma-separated room IDs; if set, bot ONLY responds in these rooms (whitelist, DMs exempt; alias of matrix.allowed_rooms)
     MATRIX_AUTO_THREAD          Auto-create threads for room messages (default: true)
     MATRIX_DM_AUTO_THREAD       Auto-create threads for DM messages (default: false)
     MATRIX_RECOVERY_KEY         Recovery key for cross-signing verification after device key rotation
@@ -343,10 +344,29 @@ class MatrixAdapter(BasePlatformAdapter):
         self._require_mention: bool = os.getenv(
             "MATRIX_REQUIRE_MENTION", "true"
         ).lower() not in ("false", "0", "no")
-        free_rooms_raw = os.getenv("MATRIX_FREE_RESPONSE_ROOMS", "")
-        self._free_rooms: Set[str] = {
-            r.strip() for r in free_rooms_raw.split(",") if r.strip()
-        }
+        free_rooms_raw = config.extra.get("free_response_rooms")
+        if free_rooms_raw is None:
+            free_rooms_raw = os.getenv("MATRIX_FREE_RESPONSE_ROOMS", "")
+        if isinstance(free_rooms_raw, list):
+            self._free_rooms: Set[str] = {
+                str(r).strip() for r in free_rooms_raw if str(r).strip()
+            }
+        else:
+            self._free_rooms: Set[str] = {
+                r.strip() for r in str(free_rooms_raw).split(",") if r.strip()
+            }
+        # If non-empty, bot ONLY responds in these rooms (whitelist); DMs exempt.
+        allowed_rooms_raw = config.extra.get("allowed_rooms")
+        if allowed_rooms_raw is None:
+            allowed_rooms_raw = os.getenv("MATRIX_ALLOWED_ROOMS", "")
+        if isinstance(allowed_rooms_raw, list):
+            self._allowed_rooms: Set[str] = {
+                str(r).strip() for r in allowed_rooms_raw if str(r).strip()
+            }
+        else:
+            self._allowed_rooms: Set[str] = {
+                r.strip() for r in str(allowed_rooms_raw).split(",") if r.strip()
+            }
         self._auto_thread: bool = os.getenv("MATRIX_AUTO_THREAD", "true").lower() in (
             "true",
             "1",
@@ -1573,6 +1593,18 @@ class MatrixAdapter(BasePlatformAdapter):
 
         # Require-mention gating.
         if not is_dm:
+            # allowed_rooms check (whitelist — must pass before other gating).
+            # When set, messages from rooms NOT in this whitelist are silently
+            # ignored, even if @mentioned.  DMs are already excluded above.
+            if self._allowed_rooms and room_id not in self._allowed_rooms:
+                logger.debug(
+                    "Matrix: ignoring message %s in %s — room not in "
+                    "MATRIX_ALLOWED_ROOMS whitelist",
+                    event_id,
+                    room_id,
+                )
+                return None
+
             is_free_room = room_id in self._free_rooms
             in_bot_thread = bool(thread_id and thread_id in self._threads)
             if self._require_mention and not is_free_room and not in_bot_thread: