Keep Go's build cache in check builds: re-vendor sneak/prompts e3f79c4 (closes #129)

The vendored files are copies from sneak/prompts e3f79c4, with this
repository's own entries kept. The script/ builds pass a new CACHEBUST
value instead of --no-cache, which emptied Go's build cache mount, so
every make test compiled the standard library and every dependency with
-race. Every Dockerfile stage declares ARG CACHEBUST first, so every RUN
step still reruns on an unchanged tree. The steps that compile mount
Go's build cache, and both go test runs keep -count=1. The lint and test
builds write no image. script/bootstrap, script/fmt and script/fmt-check
are the canonical files plus this repository's Go steps.

Model: opus-5-5

README.md

@@ -675,6 +675,10 @@ provide:
 - `script/install-precommit` — install the git pre-commit hook that runs
   `script/precommit`
 
+Every stage of the `Dockerfile` declares `ARG CACHEBUST` as its first line after
+`FROM`. The `script/` builds pass a new `CACHEBUST` value on every build, which
+reruns every `RUN` step and keeps Go's build cache; `--no-cache` would empty it.
+
 ## Features
 
 - **Multiple Authentication Methods**: Supports passphrase and PGP unlockers