feat(FN-4153): complete Step 1 — add ephemeralAgentsEnabled project setting

Fusion-Task-Id: FN-4153
Fusion-Task-Lineage: 6ec09467-c7a0-4a9d-9a09-16d5ef6f7a86

packages/core/src/__tests__/settings-parity.test.ts

@@ -79,6 +79,12 @@ describe("settings key parity", () => {
     expect(isGlobalSettingsKey("executorAllowSiblingBranchRename")).toBe(false);
   });
 
+  it("defaults ephemeralAgentsEnabled to true and keeps it project-scoped", () => {
+    expect(DEFAULT_PROJECT_SETTINGS.ephemeralAgentsEnabled).toBe(true);
+    expect(isProjectSettingsKey("ephemeralAgentsEnabled")).toBe(true);
+    expect(isGlobalSettingsKey("ephemeralAgentsEnabled")).toBe(false);
+  });
+
   it("defaults completionDocumentationMode to off", () => {
     expect(DEFAULT_PROJECT_SETTINGS.completionDocumentationMode).toBe("off");
   });

packages/core/src/settings-schema.ts

@@ -222,6 +222,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   verificationFixRetries: 3,
   buildTimeoutMs: 300_000,
   requirePlanApproval: false,
+  ephemeralAgentsEnabled: true,
   agentProvisioning: {},
   specStalenessEnabled: false,
   specStalenessMaxAgeMs: 6 * 60 * 60 * 1000,

packages/core/src/types.ts

@@ -2248,6 +2248,12 @@ export interface ProjectSettings {
    *  remain in triage with status "awaiting-approval" until a user approves
    *  or rejects the plan. Default: false. */
   requirePlanApproval?: boolean;
+  /** Controls task-worker execution mode.
+   *  - true (default): spawn short-lived `executor-FN-XXXX` ephemeral workers per task
+   *  - false: disable ephemeral workers; scheduler auto-assigns dispatchable tasks
+   *    to permanent executor agents using the reporting chain heuristic.
+   *  Tasks without an eligible permanent executor remain queued. */
+  ephemeralAgentsEnabled?: boolean;
   /** Approval policy for agent provisioning tools (fn_agent_create/fn_agent_delete). */
   agentProvisioning?: {
     approvalMode?: AgentProvisioningApprovalMode;