The stricter canonical .golangci.yml surfaced 81 findings in the
config validation code merged from main (#53). Fix them all with no
behavior change: static sentinel errors wrapped with %w preserving the
existing messages (err113), config key name constants (goconst),
t.Parallel() throughout except the Setenv/Chdir test (paralleltest),
white-box test renamed to config_validation_internal_test.go
(testpackage), case tables extracted into builder functions plus a
shared runAbortCases helper (funlen/dupl/gochecknoglobals), plain
error assignments (noinlineerr), any instead of interface{} and
strings.SplitSeq (modernize), slog.DiscardHandler (sloglint), 88-col
wrapping (lll), and removal of two stale nolint:gosec directives
(nolintlint).
