fix: resolve golangci-lint exclusions and clean up pre-existing lint issues

[tekenstam]

Background

When golangci-lint v2 was introduced in #542, several pre-existing staticcheck issues were found and suppressed via exclusion rules in `.golangci.yml` rather than fixed, to keep the PR focused. This issue tracks removing those exclusions by fixing the underlying problems.

The exclusion rules to remove are in `.golangci.yml` under `linters.exclusions.rules`.

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Issues to Fix

`staticcheck` — QF1008, ST1011, QF1006

Code	Location	Description
QF1008	`controllers/cloud.go:53,59,65`	Remove unnecessary embedded field qualifier `AmazonClientSet` from selectors
QF1008	`controllers/rollingupgrade_controller.go:87`	Remove unnecessary embedded field qualifier `NamespacedName`
QF1008	`controllers/rollingupgrade_controller.go:319`	Remove unnecessary embedded field qualifier `Client`
QF1008	`controllers/upgrade_test.go:388`	Remove unnecessary embedded field qualifier `KubernetesClientSet`
ST1011	`controllers/providers/kubernetes/nodes.go:43`	`var PostDrainDelaySeconds` is of type `time.Duration` — don't use unit-specific suffix `Seconds`; rename to `PostDrainDelay`
QF1006	`controllers/upgrade.go:561`	Lift condition into loop

QF1008 — these are simple selector simplifications, e.g.:
```go
// Before
r.AmazonClientSet.AutoScaling.DescribeXxx(...)
// After
r.AutoScaling.DescribeXxx(...)
```

ST1011 — rename `PostDrainDelaySeconds` → `PostDrainDelay` (update all references).

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Definition of Done

• All items above are fixed
• The `rules:` section is removed from `.golangci.yml`
• `golangci-lint run ./...` passes with no exclusion rules