feat(matrix): add group policy and strict mention gating

nanobot/config/schema.py

@@ -1,6 +1,8 @@
 """Configuration schema using Pydantic."""
 
 from pathlib import Path
+from typing import Literal
+
 from pydantic import BaseModel, Field, ConfigDict
 from pydantic.alias_generators import to_camel
 from pydantic_settings import BaseSettings
@@ -73,6 +75,9 @@ class MatrixConfig(Base):
     # Max seconds to wait for sync_forever to stop gracefully before cancellation fallback.
     sync_stop_grace_seconds: int = 2
     allow_from: list[str] = Field(default_factory=list)
+    group_policy: Literal["open", "mention", "allowlist"] = "open"
+    group_allow_from: list[str] = Field(default_factory=list)
+    allow_room_mentions: bool = False
 
 
 class EmailConfig(Base):