fix: use context.Background() for watcher goroutine lifetime

The OnStart hook previously derived the watcher's context from startCtx
via context.WithoutCancel(). While WithoutCancel strips cancellation,
using context.Background() makes the intent explicit: the watcher's
monitoring loop must outlive the fx startup phase and is controlled
solely by the cancel func called in OnStop.

Closes #53

internal/watcher/watcher.go

@@ -72,13 +72,15 @@ func New(
 	}
 
 	lifecycle.Append(fx.Hook{
-		OnStart: func(startCtx context.Context) error {
-			ctx, cancel := context.WithCancel(
-				context.WithoutCancel(startCtx),
-			)
+		OnStart: func(_ context.Context) error {
+			// Use context.Background() — the fx startup context
+			// expires after startup completes, so deriving from it
+			// would cancel the watcher immediately. The watcher's
+			// lifetime is controlled by w.cancel in OnStop.
+			ctx, cancel := context.WithCancel(context.Background())
 			w.cancel = cancel
 
-			go w.Run(ctx)
+			go w.Run(ctx) //nolint:contextcheck // intentionally not derived from startCtx
 
 			return nil
 		},