feat(command/healthcheck): gate client selection on feature flag

Adds feature-flag dispatch to gitlab-shell-check: when the use_new_healthcheck_client Flipt flag is enabled, the command uses the new gitlab.HealthcheckClient; otherwise it falls back to the legacy gitlabnet/healthcheck client.

Uses the FeatureFlagEvaluatorFromContext already wired into Setup(). The targeting key is set to "healthcheck" (a constant, per-process grain) so Flipt's provider doesn't bail with TargetingKeyMissingResolutionError.

Promotes github.com/open-feature/go-sdk from indirect to direct in go.mod.

Test plan

• go test ./internal/command/healthcheck/...

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Stack (merge in order)

#	MR	Status
1	!1465 (merged) — retry policy	merge into main first
2	!1466 (merged) — new HealthcheckClient	retarget to main after MR1 merges
3	!1467 (merged) — FF-gated dispatch ← you are here	retarget to main after MR2 merges
4	!1468 (merged) — acceptance test harness	retarget to main after MR3 merges