Set the lint digest and .golangci.yml together; state the Go version rule

The canonical .golangci.yml now names exhaustruct_v5, which v2.12.2
rejects, so a repo that re-vendors the file on the old digest gets a
lint phase that fails before checking any code. REPO_POLICIES now says
a repo sets the lint phase digest and re-vendors .golangci.yml in one
commit, whichever prompted the change, and both repo checklists point
to that rule. It also replaces "a new Go version needs a golangci-lint
built with it" with the rule golangci-lint applies: the go directive
must not name a newer Go minor version than the one it was built with.

Model: opus-5-5

TODO.md

@@ -25,7 +25,9 @@ fmt-check, and commit.
   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`.
+  `.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.
 - 2026-10-02: The image version now comes from git inside the build (issues 69
   and 71), superseding the 2026-09-08 entry that excluded `.git`. The canonical
   `.dockerignore` sends `.git` but keeps out `.git/config`, which can hold a