Use informer lister, context-aware shutdown, and standard logging

[jenting]

Summary

The three deferred code improvements to main.go. Behaviour-preserving except where noted (the lister sort fixes a latent spurious-restart bug).

Informer lister instead of repeated List calls

• updateConfigMap previously issued a fresh Namespaces().List() API call on every informer event. It now reads from the informer's cached lister — the data is already in memory.
• Sort the namespace list. The lister iterates a map-backed cache in non-deterministic order; without sorting, the comma-joined value could change between events even when the namespace set is identical, flipping the ConfigMap and triggering an unnecessary main-container restart. Sorting makes the value stable.

Context-aware graceful shutdown

• Replaced the ad-hoc stopCh + signalCh + os.Exit(0) (which skipped deferred cleanup) with signal.NotifyContext.
• The context is threaded through the ConfigMap Get/Update/Create calls (no more context.TODO()), so in-flight requests are cancelled on shutdown.
• A cache-sync failure is now a fatal startup error instead of being silently ignored.

Standard logging

• Switched fmt.Printf/fmt.Println logging to the standard log package (timestamps), and log.Fatalf for startup failures.

Tests

main_test.go now builds and syncs a namespace lister from the fake clientset (mirroring production) and asserts the sorted ordering. All existing assertions retained, including the nil-map panic regression test.

Verification (local)

• gofmt -l . → clean
• go vet ./... ✅
• make test ✅ (4 tests pass)
• make build (-race) ✅

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