refactor(sdk): drop message content from TypeScript 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/typescript/__tests__/evidence.unit.test.ts

@@ -26,7 +26,6 @@ const EVIDENCE: Evidence = {
       id: 'msg-sentinel',
       session_id: 'session-1',
       peer_id: 'alice',
-      content_preview: 'I drink a lot of coffee',
       created_at: '2026-01-01T00:00:00Z',
     },
   ],

sdks/typescript/src/types/api.ts

@@ -89,12 +89,17 @@ export interface EvidenceObservation {
   source_ids: string[]
 }
 
-/** 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.
+ */
 export interface EvidenceMessageRef {
   id: string
   session_id: string
   peer_id: string
-  content_preview: string
   created_at: string
 }