Simplify the Manager API and code

[pior]

Rewrite Manager: DAG → two-tier model

Replace the DAG-based AppManager with a simpler two-tier Manager that separates processes (foreground work) from services (infrastructure). On shutdown, processes are stopped first, then services.

New API

m := runnable.Manager().Name("app").ShutdownTimeout(30 * time.Second)
m.RegisterService(runnable.Closer(db))
m.Register(runnable.HTTPServer(server))
m.Register(jobWorker)
runnable.Run(m)

• Manager() — constructor, returns a Runnable
• .Register(...) — register processes (cancelled first during shutdown)
• .RegisterService(...) — register services (cancelled after all processes stop)
• .Name(string) — set a custom log prefix (default: "manager")
• .ShutdownTimeout(dur) — set deadline per shutdown phase (default: 10s)
• ManagerRegistry — interface exposing Register/RegisterService for passing to helper functions

What changed

• manager.go — full rewrite. Two-tier run logic with context.WithoutCancel, event-driven shutdown via buffered channels (no polling ticker), two-phase cancellation
• manager_container.go — deleted. No more per-runnable isolated contexts or DAG tracking
• manager_test.go — 6 tests: cancellation, empty manager, dying process, dying service, shutdown timeout, shutdown ordering
• run.go — RunGroup simplified to use Manager().Register()
• example_test.go, examples/ — updated to new API
• README.md — updated Manager section

Removed

• AppManager interface, Build(), Add(runnable, ...dependencies)
• ManagerOption / ManagerShutdownTimeout() (replaced by fluent .ShutdownTimeout())
• managerContainer, managerContainerSet (entire file)
• Polling-based shutdown loop with 10ms ticker