feat: add agents.defaults.compaction.notifyUser config option (default: false) [Fix #54249] (#54251)

Merged via squash.

Prepared head SHA: 6fd4cdb7c3
Co-authored-by: oguricap0327 <266246182+oguricap0327@users.noreply.github.com>
Co-authored-by: jalehman <550978+jalehman@users.noreply.github.com>
Reviewed-by: @jalehman

src/config/types.agent-defaults.ts

@@ -354,6 +354,11 @@ export type AgentCompactionConfig = {
    * Default: false (existing behavior preserved).
    */
   truncateAfterCompaction?: boolean;
+  /**
+   * Send a "🧹 Compacting context..." notice to the user when compaction starts.
+   * Default: false (silent by default).
+   */
+  notifyUser?: boolean;
 };
 
 export type AgentCompactionMemoryFlushConfig = {