refactor: replace primary/secondary engine pattern with uniform ProjectEngineManager

Remove the anti-pattern where the cwd project was treated as "primary" with a
special engine, and other projects got "secondary" engines through a separate
code path. Every project now gets an identical ProjectEngine created through
ProjectEngineManager.

Key changes:
- Add ProjectEngineManager class to @fusion/engine for uniform engine lifecycle
- Replace manual engine maps in dashboard.ts and serve.ts with engineManager
- Add engineManager to ServerOptions for per-project engine resolution
- Add getProjectContext() helper in routes.ts (replaces 199 getScopedStore calls)
- Merge and automation routes now resolve engine subsystems per-request
- SSE endpoint uses engine's store when available (same EventEmitter)
- Fix tsx not found in dev-with-memory.mjs startup script
- Add invalidateAllGlobalSettingsCaches for cross-project settings sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/engine/src/__tests__/project-engine-manager.test.ts

@@ -3,7 +3,8 @@ import { describe, it, expect, vi, beforeEach } from "vitest";
 // Mock ProjectEngine before importing the manager
 vi.mock("../project-engine.js", () => {
   return {
-    ProjectEngine: vi.fn().mockImplementation((config, _centralCore, _options) => ({
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    ProjectEngine: vi.fn().mockImplementation((config: any) => ({
       start: vi.fn().mockResolvedValue(undefined),
       stop: vi.fn().mockResolvedValue(undefined),
       getTaskStore: vi.fn().mockReturnValue({ projectId: config.projectId }),
@@ -127,6 +128,7 @@ describe("ProjectEngineManager", () => {
       // Make the first start fail
       let callCount = 0;
       (ProjectEngine as unknown as ReturnType<typeof vi.fn>).mockImplementation(
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
         (config: any) => ({
           start: vi.fn().mockImplementation(async () => {
             callCount++;
@@ -260,6 +262,7 @@ describe("ProjectEngineManager", () => {
 
       const store = manager.getStore("proj_aaa");
       expect(store).toBeDefined();
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
       expect((store as any).projectId).toBe("proj_aaa");
     });