feat(FN-2115): add heartbeat multiplier and agent interval controls

- Add heartbeatIntervalMultiplier to shared settings schema/types with settings parity coverage
- Apply heartbeat multiplier in engine scheduling logic while preserving explicit per-agent interval behavior
- Add Settings modal and Agents view controls for heartbeat multiplier and per-agent interval overrides, including new styling
- Expand dashboard and engine test coverage for multiplier/heartbeat controls and document the new setting

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

@@ -49,9 +49,14 @@ describe("settings key parity", () => {
     expect(isGlobalSettingsKey("themeMode")).toBe(true);
     expect(isGlobalSettingsKey("maxConcurrent")).toBe(false);
     expect(isProjectSettingsKey("maxConcurrent")).toBe(true);
+    expect(isProjectSettingsKey("heartbeatMultiplier")).toBe(true);
     expect(isProjectSettingsKey("themeMode")).toBe(false);
   });
 
+  it("includes heartbeatMultiplier in project defaults", () => {
+    expect(DEFAULT_PROJECT_SETTINGS.heartbeatMultiplier).toBe(1);
+  });
+
   it("No key appears in both GLOBAL_SETTINGS_KEYS and PROJECT_SETTINGS_KEYS", () => {
     const projectKeySet = new Set(PROJECT_SETTINGS_KEYS as readonly string[]);
     const overlap = (GLOBAL_SETTINGS_KEYS as readonly string[]).filter((key) => projectKeySet.has(key));

packages/core/src/settings-schema.ts

@@ -59,6 +59,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   globalMaxConcurrent: 4,
   maxWorktrees: 4,
   pollIntervalMs: 15000,
+  heartbeatMultiplier: 1,
   groupOverlappingFiles: true,
   autoMerge: true,
   mergeStrategy: "direct",

packages/core/src/types.ts

@@ -1033,6 +1033,10 @@ export interface ProjectSettings {
   globalMaxConcurrent?: number;
   maxWorktrees: number;
   pollIntervalMs: number;
+  /** Global multiplier applied to all agent heartbeat intervals.
+   *  For example, 0.5 halves the interval (faster checks), 2.0 doubles it (slower checks).
+   *  Must be > 0. Default: 1 (no change). */
+  heartbeatMultiplier?: number;
   groupOverlappingFiles: boolean;
   autoMerge: boolean;
   /** How completed in-review tasks should be finalized when autoMerge is enabled.