chore(lint): drop all //nolint:ireturn directives via allow-list

ireturn directives fought with gofmt/nolintlint: depending on the golangci-lint version the directive was either flagged as unused or required again. Keep the linter enabled (still guards new code) but move the 7 inline directives into the .golangci.yaml allow-list: bubbletea/v2.Msg, huh/v2.Theme, checkpoint.CommittedStore, checkpoint.TemporaryStore. summarize.Generator was already allowed.