golangci-lint v2 removed the --out-format CLI flag, causing the lint
job to fail. The golangci-lint-action v7 already uses problem matchers
to surface issues natively in GitHub Actions.
Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.