feat(discord): add channel_prompts config

Add native Discord channel_prompts support with parent forum fallback,
ephemeral runtime injection, config migration updates, docs, and tests.

gateway/platforms/base.py

@@ -682,6 +682,10 @@ class MessageEvent:
     # Auto-loaded skill(s) for topic/channel bindings (e.g., Telegram DM Topics,
     # Discord channel_skill_bindings).  A single name or ordered list.
     auto_skill: Optional[str | list[str]] = None
+
+    # Per-channel ephemeral system prompt (e.g. Discord channel_prompts).
+    # Applied at API call time and never persisted to transcript history.
+    channel_prompt: Optional[str] = None
     
     # Internal flag — set for synthetic events (e.g. background process
     # completion notifications) that must bypass user authorization checks.