Bump the pinned golangci-lint from v2.12.1 to v2.12.2 in the
Dockerfile lint stage (tagged, digest-pinned Debian image) and in
script/bootstrap (go install ref). Replace .golangci.yml with the
canonical config: linter settings (lll, funlen, cyclop, dupl) move
under linters.settings per the v2 schema so they are actually
applied, and the redundant issues.exclude-use-default key is
dropped. No new lint findings surfaced; make check is green.
