fix: use ListAccountsPages instead of manual pagination loop

[stark256-spec]

Closes #21.

Change

Replaces the hand-rolled NextToken pagination loop in listAccountsForMaster() with Organizations.ListAccountsPages(), which handles pagination automatically.

Before (~20 lines of manual pagination):

result, err := as.organizationsSvc.ListAccounts(input)
// ...
for token != "" {
    input.NextToken = &token
    result, err := as.organizationsSvc.ListAccounts(input)
    // ...
}

After (idiomatic SDK pagination):

err := as.organizationsSvc.ListAccountsPages(
    &organizations.ListAccountsInput{},
    func(page *organizations.ListAccountsOutput, lastPage bool) bool {
        accounts = append(accounts, page.Accounts...)
        return !lastPage
    },
)

The test mock is updated from ListAccounts to ListAccountsPages to match the new call site.