Re-vendor the canonical files from sneak/prompts at c55a0cb (closes #95)

Every vendored file, REPO_POLICIES.md and every model script is the
copy at sneak/prompts c55a0cb, with this repository's own entries kept
after the canonical content. Lint and test are phases of the Dockerfile
that write no image, built uncached. make test runs the suite under the
race detector as nobody, because root reads the files the tests make
unreadable. Dockerfile.lint, script/verify-lint-image-pin and
make test-race are gone. Prettier runs on the host, from the node and
yarn that script/bootstrap installs. golangci-lint v2.14.0 raises no
findings. .claude/settings.json is deleted.

Deviation: the set comes from c55a0cb on next rather than dd4027b, as
the instructions on sneak/prompts#78 allow.

Model: opus-5-5

TODO.md

@@ -46,9 +46,8 @@
   https://git.eeqj.de/sneak/sfdupes/issues/12)
 
 - cut the narration from `TODO.md` Completed Steps and from the comments in
-  `script/`, `Dockerfile` and `Dockerfile.lint` (gone since
-  https://git.eeqj.de/sneak/sfdupes/issues/95); §Workflow now branches from and
-  merges to `next` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/49)
+  `script/` and both Dockerfiles; §Workflow now branches from and merges to
+  `next` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/49)
 
 - `make test-race` ran the test suite under the race detector in a cgo-enabled
   container, outside `make check` (2026-10-04,
@@ -177,45 +176,82 @@
 
 - fix the lint-image pin comments and `FROM` form in `Dockerfile` and
   `Dockerfile.lint` (2026-08-10, branch `next`, closes
-  https://git.eeqj.de/sneak/sfdupes/issues/25): both pins became the policy
+  https://git.eeqj.de/sneak/sfdupes/issues/25): both pins are now the policy
   `# image:vX.Y.Z, YYYY-MM-DD` comment over a bare `FROM image@sha256:...`,
-  without the false `(Debian-based)` note or the tag. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 the `Dockerfile`'s `lint` phase
-  holds the only golangci-lint pin, in that form, so `Dockerfile.lint` and
-  `script/verify-lint-image-pin`, which compared the two pins, are gone.
+  without the false `(Debian-based)` note or the tag; digest unchanged.
+  `script/verify-lint-image-pin` still matches the tagless form, and a tag on
+  one side only is caught as a plain mismatch.
 
 - run all linting in Docker via `Dockerfile.lint` and `script/lint` (2026-08-10,
   branch `next`, closes https://git.eeqj.de/sneak/sfdupes/issues/46): per the
-  owner ruling the linter is never installed on a host, and
-  `golangci-lint config verify` runs before `golangci-lint run`.
-  `script/bootstrap` stopped installing or pinning the linter, and
-  `script/verify-linter-pin` was retired. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 both commands run in the
-  `Dockerfile`'s `lint` phase, which `script/lint` builds alone and the build
-  stage depends on through `COPY --from=lint /src/go.sum /dev/null`;
-  `Dockerfile.lint` and `script/verify-lint-image-pin` are gone. Nothing inside
-  an image build may run docker, so the phase calls `golangci-lint` directly.
+  owner ruling the linter is never installed on a host. `Dockerfile.lint` copies
+  the repo into the digest-pinned `golangci/golangci-lint:v2.12.2` image and
+  runs `golangci-lint config verify` and `golangci-lint run` as build steps;
+  `script/lint` builds it. `script/bootstrap` no longer installs or pins the
+  linter, and warns rather than fails when `docker` is absent;
+  `ENV PATH=/home/builder/go/bin:$PATH` went with its `go install`.
+  `script/verify-linter-pin` is retired; `script/verify-lint-image-pin`, a gate
+  in both files, compares their two `FROM` lines and restates neither pin.
+  Traps: nothing inside an image build may shell out to docker, so the
+  `Dockerfile` lint stage calls `golangci-lint` directly and the build stage
+  runs `make test` and `make fmt-check` instead of `make check`, through `make`
+  because the Makefile's `export CGO_ENABLED = 0` only reaches what it invokes.
+  `COPY --from=lint /src/go.sum /dev/null` replaces the copied linter binary as
+  the only edge making the build stage wait for lint; dropping it would end
+  fail-fast linting under a still-green build. `golangci-lint config verify`,
+  included per the ruling, validates from an embedded schema with no network
+  call, but `go mod download` above the gates still needs the network on a cold
+  cache. Verified: `make lint` green with no `golangci-lint` on `PATH`; two
+  back-to-back `script/lint` runs on an untouched tree both ran the linter
+  (27.7s and 28.7s in the lint step, `COPY . .` `CACHED` above); a planted
+  unused variable failed `script/lint`, and failed `make docker` at `[lint 9/9]`
+  with the build stage stopped at `[builder 3/12]`; the drift guard fails on a
+  tag-only, a digest-only and an unreadable reference, naming both sides; under
+  `--network none` config verify passes a valid config and rejects an invalid
+  one; `make docker` green in 5m35s with all six gates run (lint 37.6s, test
+  25.2s reporting `ok sneak.berlin/go/sfdupes 1.938s coverage: 88.5%`, not
+  `(cached)`); in the builder image with the Go test cache off, `--user 0:0`
+  still fails `TestScanHardlinkRunFailsTogether` where the unprivileged user
+  passes. Noted for follow-up, not fixed here: `golangci-lint` warns that
+  `gomodguard` is deprecated since v2.12.0 in favour of `gomodguard_v2`.
 
 - install the Docker build stage's prerequisites by running `script/bootstrap`
   instead of `apk add --no-cache make` inline (2026-08-09, branch
-  `dockerfile-bootstrap`, closes https://git.eeqj.de/sneak/sfdupes/issues/42),
-  with `script/verify-linter-pin` failing the build unless the linter copied
-  from the lint stage was the version `script/bootstrap` pinned. Builds then
-  took up to 5m14s cold, mostly in a `chown -R` of the module cache, filed as
-  https://git.eeqj.de/sneak/sfdupes/issues/43. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 the build stage installs `git` and
-  `make` with `apk add --no-cache` and only compiles; the `lint` and `test`
-  phases are the gates
+  `dockerfile-bootstrap`, closes https://git.eeqj.de/sneak/sfdupes/issues/42):
+  the stage copies `script/` plus `go.mod`/`go.sum` and runs `script/bootstrap`,
+  which ends in `go mod download`, so the separate call to it is gone.
+  `COPY --from=lint /usr/bin/golangci-lint` stays and moves above the bootstrap
+  layer: it is the only edge making this stage depend on the lint stage, so
+  deleting it would end fail-fast linting silently. A new
+  `script/verify-linter-pin`, run in the build stage before bootstrap, fails the
+  build naming both versions unless that copied binary is the version
+  `script/bootstrap` pins; a pin it cannot read is a hard failure, not a skip.
+  `$GOPATH/bin` joins `PATH`, where bootstrap's `go install` lands. The `chown`
+  and `USER builder` still precede `make check`. Verified: the guard fails the
+  build with both versions named when the lint stage's linter is faked to
+  another version, and passes an unmodified build; bootstrap runs clean under
+  Alpine's `sh` and `apk`, finding the copied linter already at the pin; a
+  second build served the bootstrap and dependency layers `CACHED` while both
+  gates ran; a planted `unused` finding failed the build at the lint gate in
+  48.9s with the build stage's `make check` never starting; and the suite run in
+  the image as `--user 0:0` fails `TestScanHardlinkRunFailsTogether`, so the
+  drop to the unprivileged user is still needed. That last check needs the Go
+  test cache off: as root it first reported `ok ... (cached)`, reusing the
+  build-time result. Build times on a noisy shared host: 2m13s on an unchanged
+  tree, 2m17s and 4m29s after a source change, 5m14s cold, which breaches the
+  policy ceiling; `chown -R builder:builder /src /home/builder` walks the module
+  cache and alone varied from 77s to 210s across those builds, and `main`
+  measured 5m03s cold with a 209s `chown`. Filed as
+  https://git.eeqj.de/sneak/sfdupes/issues/43
 - bust the Docker layer cache for the gate steps, so `script/cibuild` and
   `script/docker` cannot report a green they did not earn (2026-08-09, branch
   `cibuild-cache-bust`, closes https://git.eeqj.de/sneak/sfdupes/issues/32): the
   `Dockerfile` copies the tree before its gates, so on an unchanged tree Docker
-  served them from cache and the build exited 0 having run nothing. The fix was
-  a `CHECK_EPOCH` build argument; since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 every `docker build` in `script/`
-  passes `--no-cache` instead. Run as root, the tests fail
+  served them from cache and the build exited 0 having run nothing. Every
+  `docker build` in `script/` now passes `--no-cache` instead
+  (https://git.eeqj.de/sneak/sfdupes/issues/95). Run as root, the tests fail
   `TestScanHardlinkRunFailsTogether`, because root reads through the `chmod(0)`
-  the test relies on, so the `test` phase runs them as `nobody`
+  the test relies on, so they run as an unprivileged user
 - check the installed golangci-lint version in `script/bootstrap` instead of
   only its presence (2026-08-09, branch `bootstrap-version-check`, closes
   https://git.eeqj.de/sneak/sfdupes/issues/24): the version lives only in