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/index.ts

@@ -36,6 +36,7 @@ export { SelfHealingManager, type SelfHealingOptions } from "./self-healing.js";
 export { PluginRunner, type PluginRunnerOptions } from "./plugin-runner.js";
 export { ProjectManager } from "./project-manager.js";
 export { ProjectEngine, type ProjectEngineOptions } from "./project-engine.js";
+export { ProjectEngineManager, type EngineManagerOptions } from "./project-engine-manager.js";
 export { NodeHealthMonitor } from "./node-health-monitor.js";
 export { PeerExchangeService, type PeerExchangeServiceOptions, type SyncResult } from "./peer-exchange-service.js";
 export { RemoteNodeClient } from "./runtimes/remote-node-client.js";