Re-vendor the canonical files from sneak/prompts at e3f79c4 (closes #95)

Every vendored file, REPO_POLICIES.md and every model script is the
copy at sneak/prompts e3f79c4, with this repository's own entries kept
after the canonical content. Lint and test are phases of the Dockerfile
that write no image. Every build in script/ passes a new CACHEBUST
value, which every stage declares first, so every RUN step runs again;
the steps that compile keep Go's build cache, and both go test runs
pass -count=1. make test runs the suite under the race detector as
nobody. Dockerfile.lint, script/verify-lint-image-pin and
make test-race are gone. Prettier runs on the host.
.claude/settings.json is deleted.

Deviation: the set comes from e3f79c4 on next rather than dd4027b.
Judgement call: the test phase mounts the cache at nobody's
/tmp/.cache/go-build, owned by nobody.

Model: opus-5-5

.editorconfig

@@ -13,3 +13,6 @@ indent_style = tab
 
 [*.go]
 indent_style = tab
+
+# This repository's own sections, such as one for another language it
+# uses, go below this comment, and a re-vendor keeps them.