[shell-operator] refactor for using as a lib

[ldmonster]

Summary

This PR makes shell-operator importable as a Go library while keeping the
standalone binary fully functional. The two key entry points — construction and
lifecycle — are now clean, composable API surfaces rather than side-effectful
init-style calls.

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What changed

Unified constructor: NewShellOperator(ctx, cfg, ...Option)

• Merges the old Init(ctx, cfg, logger) + NewShellOperator(ctx, ms, hms, opts...) pair into a single function that owns the full assembly sequence (directories, metrics, debug server, kube clients, webhook managers, hook discovery).
• Init is kept as a one-line compatibility shim (Deprecated godoc, will be removed in a future release).
• NewBareShellOperator(ctx, opts...) is exposed for tests and downstream tooling that need to assemble a partial operator by hand.
• Two new option constructors, WithMetricStorage and WithHookMetricStorage, let callers share a Prometheus registry with an outer program.

Lifecycle: Start, Run, Shutdown

• Start(ctx) error is now non-blocking and idempotent (guarded by sync.Once); repeated calls return the cached error.
• Shutdown(ctx) error tears down every subsystem in reverse order — schedule manager, manager events handler, kube informers, task queues, admission/conversion webhooks, debug server, API server, and tracked goroutines — respecting the provided context deadline. Also idempotent.
• Run(ctx) error is a convenience wrapper: Start → block on ctx → Shutdown. It is the recommended entry point for main().
• DefaultShutdownTimeout = 30s replaces the old WaitQueuesTimeout = 10s package variable.
• DebugShutdowner interface decouples operator.go from pkg/debug, keeping the debug server optional.

Goroutine hygiene

• Metric goroutines (runLiveTicks, runQueueLengthMetric) are tracked via sync.WaitGroup and use select-on-channel instead of time.Sleep, so they exit cleanly when the operator context is canceled.
• baseHTTPServer gains a Shutdown(ctx) error method backed by http.Server.Shutdown; doneCh lets the old ErrServerClosed log-fatal path become a structured error log.

Webhook settings: no more package-level singletons

• admission.InitFromSettings / conversion.InitFromSettings calls are replaced with admissionSettingsFromConfig / conversionSettingsFromConfig helpers that build settings structs directly from *app.Config. This removes the package-level globals that made parallel tests flaky.

Misc cleanup

• app.AppStartMessage global removed; callers compose their own banner from app.AppName + app.Version.
• taskFactory field on ShellOperator replaces the package-level globalHookTaskFactory.
• RateLimitWait now receives the task handler's ctx instead of context.Background(), so in-flight rate-limit waits are interrupted at shutdown.
• MetricNames *metrics.Names field on ShellOperator gives library consumers a stable handle to the resolved metric names without touching package globals.