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>

scripts/dev-with-memory.mjs

@@ -27,6 +27,6 @@ if (args.length === 0) {
   });
 } else {
   // Forward all arguments (e.g., "dashboard", "task list", etc.)
-  const cmd = spawn("pnpm", ["build", "&&", "tsx", "packages/cli/src/bin.ts", ...args], { stdio: "inherit", shell: true });
+  const cmd = spawn("pnpm", ["build", "&&", "pnpm", "exec", "tsx", "packages/cli/src/bin.ts", ...args], { stdio: "inherit", shell: true });
   cmd.on("close", (c) => process.exit(c ?? 1));
 }