Pin golangci-lint v2.14.0; disable exhaustruct_v5 (closes #65)

The canonical golangci-lint moves from v2.12.2 to v2.14.0, built with go1.27: v2.12.2 refuses a module whose `go` directive names 1.27 or later. The policy now states the rule: the `go` directive must not name a newer Go minor version than the one golangci-lint was built with.

From v2.13.0, `default: all` turns on `exhaustruct_v5`, the successor of the deprecated `exhaustruct`. `.golangci.yml` disables it beside the old name, which stays listed or its deprecation warning returns.

v2.12.2 rejects the new `.golangci.yml`, so a repo changes the lint phase digest and re-vendors `.golangci.yml` in one commit; both checklists point to that rule.

Model: opus-5-5

TODO.md

@@ -21,6 +21,13 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-03: Moved the canonical golangci-lint to v2.14.0, built with go1.27,
+  because v2.12.2 refuses to lint a module whose `go` directive is 1.27 (issue
+  65). Releases from v2.13.0 deprecate `exhaustruct` in favour of
+  `exhaustruct_v5`, which `default: all` switches on, so the canonical
+  `.golangci.yml` now disables `exhaustruct_v5` beside `exhaustruct`. v2.12.2
+  rejects that file, so `REPO_POLICIES.md` and both repo checklists now say a
+  repo sets the lint phase digest and re-vendors `.golangci.yml` in one commit.
 - 2026-10-03: Brought the canonical `.gitignore` level with `.dockerignore` on
   secrets (issue 38): it now also ignores `prod.env`-style `*.env` files,
   `.envrc`, `*.p12`, `*.pfx` and the extensionless SSH private keys, written to