Update golangci-lint to v2.12.2 with canonical config

- Replace .golangci.yml with the canonical v2-schema config: linter
  settings move under linters.settings (the previous top-level
  linters-settings block was ignored by golangci-lint v2, so the
  configured thresholds were not applied) and the obsolete
  issues.exclude-use-default key is dropped.
- Bump golangci-lint from the v2.10.1-era commit pin to @v2.12.2 in
  Dockerfile and script/bootstrap; refresh pin date comments.
- Wrap long lines in attrsum.go to satisfy the now-effective lll
  limit of 88 columns (15 findings); move one nolint:gosec directive
  to its own line.
- Record the change in TODO.md Completed Steps.

script/bootstrap

@@ -4,14 +4,14 @@
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes nothing is present.
 # golangci-lint and goimports are installed via `go install` at the same
-# pinned commits the Dockerfile uses (never "latest").
+# pinned refs the Dockerfile uses (never "latest").
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-07-07 (same pins as the Dockerfile)
-# golangci-lint v2.10.1
-GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@5d1e709b7be35cb2025444e19de266b056b7b7ee"
+# Pinned versions, 2026-08-07 (same pins as the Dockerfile)
+# golangci-lint v2.12.2
+GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.12.2"
 # goimports v0.42.0
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"