Commit yarn 4 in the tree and install with --immutable (closes #149)

Follows sneak's ruling at
#149 (comment). This
repository now carries yarn 4.18.1 under .yarn/releases. The
distribution's yarn only starts that file, and that file does the work.
yarn.lock is converted once, and script/bootstrap installs with
--immutable instead of stopping on yarn's major version. Node is the
distribution's newest Node package: nodejs-current on Alpine and
nodejs_latest on nix. Every Dockerfile stage that needs yarn also copies
.yarnrc.yml and .yarn/releases/. The policies and both checklists say
so, with the steps for converting and for a new repository.

Model: opus-5-5

TODO.md

@@ -21,18 +21,6 @@ fmt-check, and commit.
 
 # Completed Steps
 
-- 2026-11-06: Compacted `prompts/REPO_POLICIES.md` from 1,145 lines and 8,427
-  words to 1,066 lines and 7,812 words
-  (https://git.eeqj.de/sneak/prompts/issues/158). Every rule keeps its meaning.
-  The rules now sit under headings, the pinning rule first. Each rule that was
-  stated in several places is stated once. The Makefile copies of the Go and
-  Python test commands are gone, because the `Dockerfile` examples carry the
-  same commands. Both checklists name the `.golangci.yml` section.
-- 2026-11-06: The pinning rule in `prompts/REPO_POLICIES.md` now opens with its
-  scope: what we download and install ourselves in scripts and Dockerfiles
-  (https://git.eeqj.de/sneak/prompts/issues/154). Distribution packages are
-  outside the rule, and updating and upgrading them is allowed. Both checklists
-  say the same.
 - 2026-11-04: Each repository now commits yarn 4 and installs with
   `yarn install --immutable` (https://git.eeqj.de/sneak/prompts/issues/149), on
   sneak's ruling at

prompts/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-11-06
+last_modified: 2026-11-04
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -163,7 +163,7 @@ with your task.
     - [ ] Go: `go.mod`, `go.sum`, `.golangci.yml` (fetch from
           `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` section of `REPO_POLICIES.md`)
+          `.golangci.yml` paragraph of `REPO_POLICIES.md`)
     - [ ] JS: `package.json`, `yarn.lock`, `.yarnrc.yml`, `.yarn/releases/`,
           `.gitattributes`, `.prettierrc`, `.prettierignore` (fetch the last two
           from `https://git.eeqj.de/sneak/prompts/raw/branch/main/.prettierrc`
@@ -267,14 +267,14 @@ with your task.
 
 - [ ] Pre-commit hook is installed (`make hooks`)
 - [ ] No secrets in the repo (`.env`, keys, credentials)
-- [ ] No mutable references (tags, `@latest`) to what the repo downloads and
-      installs itself in scripts and Dockerfiles — all pinned by cryptographic
-      hash with version/date comment. Distribution packages are outside this
-      rule. They need no version and no hash, and updating and upgrading them is
-      fine too. In a `Dockerfile`, the distribution is the pinned base image's.
-      On a host, it is the host's, and the host's package manager stands for it,
-      nix and brew included. A package source that a `Dockerfile` adds itself is
-      not the distribution's own, so its packages are pinned
+- [ ] No mutable references in Dockerfiles or scripts (tags, `@latest`) — all
+      pinned by cryptographic hash with version/date comment. The one exception
+      is a package from the distribution's own package index, which needs no
+      version and no hash. Pulling that index is also allowed. In a
+      `Dockerfile`, the distribution is the pinned base image's. On a host, it
+      is the host's, and the host's package manager stands for it, nix and brew
+      included. A package source that a `Dockerfile` adds itself does not get
+      this exception
 - [ ] node, yarn, npm, python, pip and venv come from the distribution wherever
       possible, never from nvm, pyenv, corepack or `npm install -g yarn`. If the
       distribution's version is too old, use a newer distribution. node means

prompts/NEW_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: New Repo Checklist
-last_modified: 2026-11-06
+last_modified: 2026-11-04
 ---
 
 Use this checklist when creating a new repository from scratch. Follow the steps
@@ -178,7 +178,7 @@ Template files can be fetched from:
     - [ ] Go: `go mod init sneak.berlin/go/<name>`, `.golangci.yml` (fetch from
           `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` section of `REPO_POLICIES.md`)
+          `.golangci.yml` paragraph of `REPO_POLICIES.md`)
     - [ ] JS, or any repository that runs prettier through yarn: make the first
           `yarn.lock` with the steps below. `script/bootstrap` installs no yarn
           until `yarn.lock` exists, so run them with any distribution's node and
@@ -285,13 +285,12 @@ are thin shims calling them. Model scripts:
       final stage is missing its `COPY --from=` edge to the gate phases.
 - [ ] No secrets in repo, and none in the build context: enumerate a probe image
       rather than reading `.dockerignore`
-- [ ] No mutable references to what the repo downloads and installs itself in
-      scripts and Dockerfiles. Distribution packages are outside this rule. They
-      need no version and no hash, and updating and upgrading them is fine too.
-      In a `Dockerfile`, the distribution is the pinned base image's. On a host,
-      it is the host's, and the host's package manager stands for it, nix and
-      brew included. A package source that a `Dockerfile` adds itself is not the
-      distribution's own, so its packages are pinned
+- [ ] No mutable image/package references. The one exception is a package from
+      the distribution's own package index, which needs no version and no hash.
+      Pulling that index is also allowed. In a `Dockerfile`, the distribution is
+      the pinned base image's. On a host, it is the host's, and the host's
+      package manager stands for it, nix and brew included. A package source
+      that a `Dockerfile` adds itself does not get this exception
 - [ ] node, yarn, npm, python, pip and venv come from the distribution wherever
       possible, never from nvm, pyenv, corepack or `npm install -g yarn`. If the
       distribution's version is too old, use a newer distribution. node means