feat(session): add guild_id/parent_chat_id/message_id to SessionSource

Groundwork for injecting raw platform identifiers into the agent's
system prompt.  Currently only `thread_id` is exposed as a raw ID —
callers in a Discord thread had to guess `channel_id == thread_id`
(which happens to work because threads are channels in Discord's REST
API) and had no way to reference the parent channel, guild, or the
triggering message.

Adds three optional fields:

- `guild_id` — Discord guild / Slack workspace / Matrix server scope
- `parent_chat_id` — parent channel when chat_id refers to a thread
- `message_id` — ID of the triggering message (pin/reply/react)

Extends `BasePlatformAdapter.build_source()` to accept + forward them
and teaches `to_dict`/`from_dict` to serialize them.  Behaviourally a
no-op: nothing reads the fields yet and they default to None.

gateway/platforms/base.py

@@ -2440,6 +2440,9 @@ class BasePlatformAdapter(ABC):
         user_id_alt: Optional[str] = None,
         chat_id_alt: Optional[str] = None,
         is_bot: bool = False,
+        guild_id: Optional[str] = None,
+        parent_chat_id: Optional[str] = None,
+        message_id: Optional[str] = None,
     ) -> SessionSource:
         """Helper to build a SessionSource for this platform."""
         # Normalize empty topic to None
@@ -2457,6 +2460,9 @@ class BasePlatformAdapter(ABC):
             user_id_alt=user_id_alt,
             chat_id_alt=chat_id_alt,
             is_bot=is_bot,
+            guild_id=str(guild_id) if guild_id else None,
+            parent_chat_id=str(parent_chat_id) if parent_chat_id else None,
+            message_id=str(message_id) if message_id else None,
         )
     
     @abstractmethod