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

prompts/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-10-02
+last_modified: 2026-10-03
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -80,7 +80,9 @@ with your task.
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`
 - [ ] Language-specific config:
     - [ ] Go: `go.mod`, `go.sum`, `.golangci.yml` (fetch from
-          `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml`)
+          `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml` and,
+          in the same commit, set the lint phase digest to the one named in the
+          `.golangci.yml` paragraph of `REPO_POLICIES.md`)
     - [ ] JS: `package.json`, `yarn.lock`, `.prettierrc`, `.prettierignore`
           (fetch from
           `https://git.eeqj.de/sneak/prompts/raw/branch/main/.prettierrc` and