Enable depguard so a non-test file cannot import test support

Test-support code exists to hand a test privileges the program itself
must never have, so it must not be compiled into the shipped binary.
depguard now states that rule: in files that are neither tests nor
inside a package whose directory name ends in test, the denied imports
are refused.

depguard matches an import path against a list of prefixes, so it
cannot be told "any path whose last segment ends in test". A repo's own
test-support packages therefore have to be named one at a time under
deny, by full import path, and the policy paragraph now says that list
is the one part of the vendored copy a repo may extend.

Requested by sneak on
sneak/homoicon#1029 to replace a
per-repository check written as a Go test.

Model: opus-5

.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/CODE_STYLEGUIDE_GO.md

@@ -124,10 +124,15 @@ last_modified: 2026-03-18
 
 1.  Keep the `main()` function as small as possible.
 
-1.  Keep the `main` package as small as possible. Move as much code as is
-    feasible to a library package, even if it's an internal one. `main` is just
-    an entrypoint to your code, not a place for implementations. Exception:
-    single-file scripts.
+1.  Keep the `main` package as small as possible. Each `cmd/<name>/` directory
+    contains a single `main.go` whose body is one call into library code (for
+    example `os.Exit(cli.Main())` calling `internal/cli`). All CLI logic — flag
+    parsing, subcommand dispatch, argument handling, output formatting — lives
+    in `internal/` or `pkg/`, not in `cmd/`. `main` is just an entrypoint to
+    your code, not a place for implementations. Exception: single-file scripts.
+
+1.  No project logic outside `internal/` or `pkg/`. Anything in `cmd/` is a thin
+    entrypoint only.
 
 1.  HTTP HandleFuncs should be returned from methods or functions that need to
     handle HTTP requests. Don't use methods or your top level functions as

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-08-07
+last_modified: 2026-08-19
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -263,11 +263,18 @@ style conventions are in separate documents:
 - Make all changes on a feature branch. You can do whatever you want on a
   feature branch.
 
-- `.golangci.yml` is standardized and must _NEVER_ be modified by an agent, only
-  manually by the user. Fetch from
-  `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml`. The
-  canonical golangci-lint version is v2.12.2 (released 2026-05-06), installed
-  commit-pinned via
+- `.golangci.yml` is standardized. The vendored copy in a consuming repo must
+  _NEVER_ be modified by an agent: fetch it from
+  `https://git.eeqj.de/sneak/prompts/raw/branch/main/.golangci.yml` and keep it
+  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. 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`.
 
 - When pinning images or packages by hash, add a comment above the reference
@@ -387,7 +394,9 @@ style conventions are in separate documents:
   language-specific config). Everything else goes in a subdirectory. Canonical
   subdirectory names:
     - `bin/` — executable scripts and tools
-    - `cmd/` — Go command entrypoints
+    - `cmd/` — Go command entrypoints; thin only: one `main.go` per binary whose
+      body is a single call into `internal/` or `pkg/`, no project logic in
+      `cmd/`
     - `configs/` — configuration templates and examples
     - `deploy/` — deployment manifests (k8s, compose, terraform)
     - `docs/` — documentation and markdown (README.md stays in root)