Keep modules, lint cache and system packages in check builds (closes #109)

Re-vendors REPO_POLICIES.md and script/cibuild, docker, lint and test
from sneak/prompts at 0653579, on sneak's ruling in
sneak/prompts#130. In the Dockerfile's lint,
test and builder stages, the system-package install and the module
download now come before ARG CACHEBUST, so Docker keeps those layers
until the base image, the install line, go.mod or go.sum changes.
golangci-lint run also mounts golangci-lint's cache. Every step that
lints, tests or compiles still runs on every check build, and go test
keeps -count=1 and -race. README and TODO.md say the same.

Model: opus-5-5

TODO.md

@@ -29,6 +29,15 @@
 
 # Completed Steps
 
+- re-vendor `REPO_POLICIES.md` and the four scripts that pass `CACHEBUST` from
+  `sneak/prompts` `next` at `0653579`: in the `lint`, `test` and `builder`
+  stages of the `Dockerfile` the module download, and in `builder` the
+  system-package install, now come before `ARG CACHEBUST`, so Docker keeps them
+  until the base image, the install line, `go.mod` or `go.sum` changes, and
+  every step that lints, tests or compiles still comes after it;
+  `golangci-lint run` also keeps golangci-lint's cache. README says the same
+  (2026-10-18, https://git.eeqj.de/sneak/sfdupes/issues/109)
+
 - releases until 1.0 are `v0.0.N` tags on `main`'s tip, so
   `go install sneak.berlin/go/sfdupes@latest` installs `main`'s tip once it has
   passed the build; README "Install" says so, says what `@main` and `v0.0.1`
@@ -154,7 +163,11 @@
   and the build stage installs `git` and `make` itself; a new push cancels the
   workflow's older run on the same branch, and a run stops after 20 minutes;
   `.claude/settings.json` is deleted (2026-10-15,
-  https://git.eeqj.de/sneak/sfdupes/issues/95)
+  https://git.eeqj.de/sneak/sfdupes/issues/95). Since
+  https://git.eeqj.de/sneak/sfdupes/issues/109 the module download in `lint`,
+  `test` and the build stage, and the install of `git` and `make` in the build
+  stage, come before `ARG CACHEBUST`, and Docker keeps them between builds;
+  every step that lints, tests or compiles still runs on every build
 
 - with `-x`, an operand under another operand is walked on its own instead of
   being dropped when a directory from it (from its directory, for a regular
@@ -355,9 +368,9 @@
   a `CHECK_EPOCH` build argument; since
   https://git.eeqj.de/sneak/sfdupes/issues/95 every `docker build` in `script/`
   passes a new `CACHEBUST` value instead, which every stage of the `Dockerfile`
-  declares first. 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`
+  declares before every step that lints, tests or compiles. 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`
 - 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