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.
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