feat(FN-4718): complete Step 1 — add done auto-archive days setting

Fusion-Task-Id: FN-4718
Fusion-Task-Lineage: 4553c39c-c174-4993-a4ac-2c8b7d86bda2

packages/core/src/settings-schema.ts

@@ -291,6 +291,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   maintenanceIntervalMs: 300_000,
   autoArchiveDoneTasksEnabled: true,
   autoArchiveDoneAfterMs: 48 * 60 * 60 * 1000,
+  doneAutoArchiveDays: 0,
   archiveAgentLogMode: "compact",
   autoUpdatePrStatus: false,
   githubCommentOnDone: false,

packages/core/src/types.ts

@@ -2835,6 +2835,9 @@ export interface ProjectSettings {
   autoArchiveDoneTasksEnabled?: boolean;
   /** Age in milliseconds after a task enters done before auto-archive. Default: 172800000 (48h). */
   autoArchiveDoneAfterMs?: number;
+  /** Retention in integer days before done tasks are auto-archived.
+   *  0 disables this days-based override. When > 0, takes precedence over autoArchiveDoneAfterMs. */
+  doneAutoArchiveDays?: number;
   /** How much agent log content to preserve when a task is moved to cold archive storage.
    *  - "compact": deterministic summary plus a small recent-entry snapshot (default)
    *  - "full": copy the full agent.log into archive.db