refactor(sdk): drop message content from Python evidence types

Follows the server: `EvidenceMessageRef` reports identity and provenance,
not content. Callers fetch a message by id when they need its text.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

sdks/python/src/honcho/api_types.py

@@ -558,12 +558,16 @@ class EvidenceObservation(BaseModel):
 
 
 class EvidenceMessageRef(BaseModel):
-    """A message the dialectic read while answering."""
+    """A message the dialectic read while answering.
+
+    Identity and provenance only -- no content. Fetch the message by `id` when
+    you need its text; evidence is for auditing what was read, not for reading
+    messages in bulk.
+    """
 
     id: str
     session_id: str
     peer_id: str
-    content_preview: str
     created_at: datetime.datetime