Keep the context section type inside the contract file

Why: A separately exported ContextSection is unused inside the UI package and pushes the knip ratchet over its baseline.
Evidence: UI knip reported 56 unused exports against baseline 55. The section shape stays on ContextObservation.sections.
Impact: Generated frontend and UI contracts drop the extra export. JSON field names are unchanged.
Refs: .task/feature-sillytavern-review.md
Co-authored-by: Cursor <cursoragent@cursor.com>

ts/types.ts

@@ -113,7 +113,7 @@ export interface RunCompletedPayload {
   context?: ContextObservation
 }
 
-export interface ContextSection {
+interface ContextSection {
   name: string
   estTokens: number
 }