Migrate canonical .golangci.yml to gomodguard_v2 with a block list

golangci-lint v2.12.0 deprecated gomodguard, and `linters.default: all`
enables it, so every lint run in every consuming Go repo prints a
deprecation warning. Disable the old name, which is what silences the
warning; name gomodguard_v2 in linters.enable so the settings block has
a visible owner; give it a blocked module list.

The block list restates only decisions already recorded in the Go
package defaults: zerolog, the pre-fork go-redis/redis, sergi/go-diff
and hexops/gotextdiff. Recorded rejections that repos vendoring this
file still require are deliberately absent, so a re-vendor cannot
redden a repo that has done nothing wrong.

Model: opus-5

.golangci.yml

@@ -17,7 +17,6 @@ linters:
   disable:
     # Genuinely incompatible with project patterns
     - exhaustruct      # Requires all struct fields
-    - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
@@ -35,6 +34,32 @@ linters:
       max-complexity: 15
     dupl:
       threshold: 100
+    depguard:
+      # Test-support code must not be compiled into the shipped binary. A
+      # test-support package exists to hand a test privileges the program
+      # itself must never have, so a file that is not a test must not import
+      # one. Test files, and the files inside a package whose directory name
+      # ends in `test`, are where that code belongs, and are exempt.
+      #
+      # The deny list below is the one part of this file a repository is
+      # expected to extend, and the only part it may. depguard matches an
+      # import path against a list of prefixes, so it cannot be told "any path
+      # whose last segment ends in test"; a repository's own test-support
+      # packages have to be named here one at a time, by full import path,
+      # under a module path that differs from repository to repository. Add
+      # them; change nothing else.
+      rules:
+        test-support:
+          list-mode: lax
+          files:
+            - "$all"
+            - "!$test"
+            - "!**/*test/**"
+          deny:
+            - pkg: net/http/httptest
+              desc: >-
+                Test-support code belongs in test files and in packages whose
+                directory name ends in test, not in the shipped binary.
     # Only decisions already recorded in the Go package defaults are
     # listed here. Entries match the module path exactly (the default
     # match-type), so a differently-versioned module path is unaffected.

prompts/REPO_POLICIES.md

@@ -269,7 +269,11 @@ style conventions are in separate documents:
   byte-identical, so that no repo can quietly loosen its own linting. Linter
   configuration changes are made to the canonical copy in the `prompts` repo and
   reach consuming repos by re-vendoring; an agent may open a PR against
-  canonical, which only the user merges. The canonical golangci-lint version is
+  canonical, which only the user merges. One list is exempt from byte-identity,
+  because it cannot be written once for every repo: the `deny` list of the
+  `test-support` depguard rule, where a repo names its own test-support packages
+  by full import path. A repo adds entries there and changes nothing else, and a
+  re-vendor carries its entries forward. The canonical golangci-lint version is
   v2.12.2 (released 2026-05-06), installed commit-pinned via
   `go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5`.