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

prompts/REPO_POLICIES.md

@@ -35,7 +35,7 @@ style conventions are in separate documents:
     - Docker: `@sha256:...`
     - Go: the module hash in `go.sum`
     - Python: the package hash in `Pipfile.lock`
-    - npm: the integrity hash in the lockfile
+    - npm: the `checksum` in `yarn.lock`
     - GitHub Actions: `@<commit-sha>`
 
     This also means never `curl | bash` to install a tool such as rustup.
@@ -56,7 +56,8 @@ style conventions are in separate documents:
     - `apk add`
 
     These come from the distribution wherever possible:
-    - node
+    - node, meaning the distribution's newest Node package, such as
+      `nodejs-current` on Alpine 3.24
     - yarn
     - npm
     - python
@@ -69,6 +70,9 @@ style conventions are in separate documents:
     - corepack
     - `npm install -g yarn`
 
+    yarn's release file is third-party code, and git pins it. `yarn set version`
+    writes it to `.yarn/releases/` once, given an exact version.
+
     These are not trusted, so everything from them stays pinned by hash:
     - Docker Hub
     - PyPI
@@ -123,9 +127,32 @@ style conventions are in separate documents:
 
     `script/bootstrap` installs node, yarn and the node packages only when
     `yarn.lock` is in the tree. With no `yarn.lock`, it installs no node and no
-    yarn, and it does not check yarn's version. Make a repository's first
-    `yarn.lock` with yarn 1, from a distribution that packages it, such as
-    Alpine (`apk add nodejs yarn`).
+    yarn. It installs the node packages with `yarn install --immutable`. That
+    install fails when a package's `checksum` is wrong, or when `package.json`
+    is out of step with `yarn.lock`.
+
+    Each repository commits the yarn that runs. `yarnPath` in `.yarnrc.yml`
+    points at the release file in `.yarn/releases/`. The distribution's yarn
+    only starts that file, and that file does the work. Alpine packages yarn 1,
+    and Debian 13 packages yarn 4.1. The same committed yarn does the work on
+    both. Corepack never runs, and no yarn is downloaded.
+
+    `.yarnrc.yml` holds these settings:
+    - `yarnPath`
+    - `nodeLinker: node-modules`
+    - `enableTelemetry: false`, because yarn otherwise sends usage reports from
+      any host that is not CI
+
+    By default, yarn runs no dependency's install script. A dependency that must
+    run one is named in `package.json`, as
+    `"dependenciesMeta": { "<name>": { "built": true } }`.
+
+    `.gitattributes` carries `.yarn/releases/** binary`, so diffs and merges
+    treat the release file as one opaque file.
+
+    yarn 1 ignores `--immutable` and exits 0, so on a tree with no `.yarnrc.yml`
+    the install is not frozen. A repository therefore re-vendors
+    `script/bootstrap` and converts its `yarn.lock` in one commit.
 
     Debian and Ubuntu name the yarn command `yarnpkg`. `script/bootstrap`,
     `script/fmt` and `script/fmt-check` each carry the same small function that
@@ -133,18 +160,18 @@ style conventions are in separate documents:
     and `script/fmt-check` print one line saying to run `script/bootstrap`, and
     exit 1.
 
-    With `yarn.lock` in the tree, `script/bootstrap` stops with a one-line
-    message when yarn's major version is not 1, before it installs any node
-    packages. Debian 13 and Ubuntu 26.04 package yarn 4. With
-    `--frozen-lockfile`, yarn 4 refuses a yarn 1 `yarn.lock`. Without that flag,
-    it rewrites the file in its own format, with yarn 4 hashes in place of the
-    yarn 1 ones. A host whose distribution packages yarn 4 moves to one that
-    packages yarn 1, such as Alpine or Ubuntu 24.04.
+    A `Dockerfile` stage that needs yarn may start from any distribution that
+    packages node and yarn. It copies all of these before it runs
+    `script/bootstrap`:
+    - `package.json`
+    - `yarn.lock`
+    - `.yarnrc.yml`
+    - `.yarn/releases/`
 
-    A `Dockerfile` stage copies `yarn.lock` before it runs `script/bootstrap`
-    only if that stage needs yarn. A stage that needs yarn starts from an image
-    whose distribution packages yarn 1, such as Alpine. The Python and Go stages
-    below need no yarn and copy no `yarn.lock`, so they never reach the stop.
+    With only `package.json` and `yarn.lock`, the install fails. Alpine's yarn 1
+    then says to enable corepack; never do that, and copy the missing files
+    instead. The Python and Go stages below need no yarn and copy no
+    `yarn.lock`, so `script/bootstrap` installs no node and no yarn there.
 
     `script/cibuild` runs the CI build, and the Gitea workflow calls it. The
     script changes to the repo root, runs `script/bootstrap`, runs
@@ -210,8 +237,9 @@ style conventions are in separate documents:
     running `script/bootstrap`. Only system packages may be installed inline
     above it, so that check builds keep them; the `prompts` repo's own
     `Dockerfile` does this. A stage that runs `script/bootstrap` COPYs `script/`
-    and the dependency manifests it needs (`package.json` + `yarn.lock`,
-    `go.mod` + `go.sum`, etc.) before running it.
+    and the dependency manifests it needs before running it. These are examples:
+    - `package.json`, `yarn.lock`, `.yarnrc.yml` and `.yarn/releases/`
+    - `go.mod` and `go.sum`
 
 - **The lint and test gates run in Docker, as phases of the `Dockerfile`.**
   There is no separate lint file. `script/lint` and `script/test` each build one
@@ -771,6 +799,7 @@ style conventions are in separate documents:
     - editor files (`.swp`, `*~`)
     - in-repo agent scratch directories (`.claude/`)
     - `node_modules/`
+    - `.yarn/*`, with `!.yarn/releases` so that git keeps yarn's release file
     - the repo's own build outputs
 
     Fetch the standard `.gitignore` from
@@ -792,16 +821,16 @@ style conventions are in separate documents:
   Such a `.dockerignore` is more dangerous than a short file with no secret
   patterns at all, because it reads as solved and stops anyone looking. Give
   every depth-independent pattern the `**/` prefix, and leave only genuinely
-  root-anchored entries unprefixed. Those are `.claude`, and the repo's own
-  host-built binary. The binary is written `/myapp` and never `**/myapp`, which
-  would also match `cmd/myapp/` and delete the package directory from the
-  context. Matching is case-sensitive, and an all-caps copy of each pattern
-  still misses `Server.Key`. So secret names use character ranges:
-  `**/*.[kK][eE][yY]`, `**/*.[pP][eE][mM]`, and likewise for `.envrc` and the
-  extensionless SSH keys. Where such a pattern also catches something the build
-  needs, re-include it with a negation (`!docs/example.env`). Deleting the
-  pattern reopens the exposure for every other file it covers. Fetch the
-  standard `.dockerignore` from
+  root-anchored entries unprefixed. Those are `.claude`, `.yarn/*`,
+  `!.yarn/releases`, and the repo's own host-built binary. The binary is written
+  `/myapp` and never `**/myapp`, which would also match `cmd/myapp/` and delete
+  the package directory from the context. Matching is case-sensitive, and an
+  all-caps copy of each pattern still misses `Server.Key`. So secret names use
+  character ranges: `**/*.[kK][eE][yY]`, `**/*.[pP][eE][mM]`, and likewise for
+  `.envrc` and the extensionless SSH keys. Where such a pattern also catches
+  something the build needs, re-include it with a negation
+  (`!docs/example.env`). Deleting the pattern reopens the exposure for every
+  other file it covers. Fetch the standard `.dockerignore` from
   `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore` and extend
   it with the repo's own artifacts.
 
@@ -1105,7 +1134,8 @@ style conventions are in separate documents:
     - `Dockerfile`, `.dockerignore`
     - `.gitea/workflows/check.yml`
     - Go: `go.mod`, `go.sum`, `.golangci.yml`
-    - JS: `package.json`, `yarn.lock`, `.prettierrc`, `.prettierignore`
+    - JS: `package.json`, `yarn.lock`, `.yarnrc.yml`, `.yarn/releases/`,
+      `.gitattributes`, `.prettierrc`, `.prettierignore`
     - Python: `pyproject.toml`
 
 - Guidance for coding agents lives in one `AGENTS.md` at the repository root. It