fix: satisfy golangci-lint 2.13.1

The renovate bump of golangci-lint to v2.13.1 (merged in 861015ec, MR !3012 (merged)) surfaced 5 lint failures on main:

• types.go:557: gofumpt now flags redundant parentheses in (time.Time(*t)).IsZero().
• gitlab.go:936, types.go:697, workitems.go:752, workitems.go:756: 4 //nolint:goconst directives are now flagged as unused by nolintlint because goconst's stricter 2.13 defaults no longer produce findings at those sites.

This MR fixes all 5. No behavior change.

Why didn't !3012 (merged)'s pipeline catch this?

Because the golangci-lint job never ran on !3012 (merged).

The .go:base template in .gitlab-ci.yml gates every Go job — including golangci-lint — on a rules: changes: list. That list included **/*.go, go.mod, go.sum, .gitlab-ci.yml, etc., but not .tool-versions. Renovate bumps of golangci-lint, gofumpt, or the Go toolchain touch only .tool-versions, so the linter job was skipped on the very MR that changed the linter version. The pipeline went green on tests alone, auto-merged, and the failures only appeared on the next unrelated MR that touched a .go file (MR !3016 (merged)).

This MR therefore also adds .tool-versions and .golangci.yml to the .go:base changes list, so future linter/tool bumps will actually exercise the linter.