Enable depguard so a non-test file cannot import test support (#59)

`depguard` was in the disable list. It is now enabled with one rule,
`test-support`: in files that are neither test files nor inside a package whose
directory name ends in `test`, the imports named under `deny` are refused.
Canonical denies `net/http/httptest`, which serves tests only and is the same
in every repository.

This replaces `internal/testimportgate` in sneak/homoicon, a package whose only
job was to refuse a non-test Go file importing an in-module package whose last
path segment ends in `test`. sneak ruled on
sneak/homoicon#1029 that the rule moves into linter
configuration here.

depguard cannot express that rule generically. Its package lists are prefix
lists -- its own README says so, and I confirmed it: `*test`, `**test` and
`$gomod/**test` each match nothing, while a full import path matches. "In
module" is not generic either, since the module path differs per repository.
And depguard refuses a rule with no allow or deny list, so this file cannot
ship the rule pre-armed and empty for each repository to fill in.

The closest expressible rule is the one here. The file half is generic and
exact; the package half is a deny list each repository extends with its own
test-support packages, by full import path. REPO_POLICIES.md now says that list
is the one part of a vendored copy a repository may add to.

Tested with golangci-lint v2.12.2 on a scratch module: a production file
importing a denied `*test` package fails, and the same import from a `_test.go`
file and from inside the `*test` package passes. `make check` here is green.

Model: opus-5
Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #59
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

.golangci.yml

@@ -13,7 +13,6 @@ linters:
   disable:
     # Genuinely incompatible with project patterns
     - exhaustruct      # Requires all struct fields
-    - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
     - wsl              # Deprecated, replaced by wsl_v5
     - wrapcheck        # Too verbose for internal packages
@@ -28,6 +27,32 @@ 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.
 
 issues:
   max-issues-per-linter: 0

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`.