[WIP] Add tidy-check Makefile target and CI step for go.mod/go.sum drift detection (#2456)

- [x] Add `tidy-check` Makefile target
- [x] Add `tidy-check` to `.PHONY` list in Makefile
- [x] Add `tidy-check` to `help` target in Makefile
- [x] Add CI step in `lint` job (between gofmt/vet check and
golangci-lint)
- [ ] Run `make agent-finished` to verify everything is correct

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Author: lpcox