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.

.golangci.yml

@@ -1,5 +1,9 @@
 version: "2"
 
+# Config schema uses the golangci-lint v2 layout (settings live under
+# linters.settings, not top-level linters-settings) so that the
+# thresholds below are actually applied by golangci-lint >= v2.
+
 run:
   timeout: 5m
   modules-download-mode: readonly
@@ -14,19 +18,17 @@ linters:
     - wsl              # Deprecated, replaced by wsl_v5
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
-
-linters-settings:
-  lll:
-    line-length: 88
-  funlen:
-    lines: 80
-    statements: 50
-  cyclop:
-    max-complexity: 15
-  dupl:
-    threshold: 100
+  settings:
+    lll:
+      line-length: 88
+    funlen:
+      lines: 80
+      statements: 50
+    cyclop:
+      max-complexity: 15
+    dupl:
+      threshold: 100
 
 issues:
-  exclude-use-default: false
   max-issues-per-linter: 0
   max-same-issues: 0