feat(llm): add default context window fallback

docs/config-catalog.md

@@ -535,6 +535,8 @@ export interface Config {
   thinking?: 'enabled' | 'disabled'
   /** Thinking effort (only meaningful with thinking enabled). */
   reasoningEffort?: 'high' | 'max'
+  /** Positive context capacity used when the selected model has no exact value. */
+  defaultContextWindow?: number
   /** Advisory models shown by discovery consumers; defaults to V4 Flash and V4 Pro. */
   models?: DeepSeekCatalogModel[]
   /** Maximum provider idle time while one stream read is outstanding (default five minutes). */