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 Dockerfile phases that
write no image; make test runs the suite under the race detector as
nobody. 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.
Dockerfile.lint, script/verify-lint-image-pin and make test-race are
gone, prettier runs on the host, and .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

script/precommit

@@ -1,13 +1,13 @@
 #!/bin/sh
 # script/precommit: run by the git pre-commit hook; fails the commit if
-# checks fail. Our own extension to scripts-to-rule-them-all. Go extra:
-# go mod tidy must be a no-op before the checks run.
+# checks fail. Our own extension to scripts-to-rule-them-all.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
 ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
+    # Go extra: go mod tidy must be a no-op before the checks run.
     cd "$ROOT"
     go mod tidy
     if ! git diff --exit-code -- go.mod go.sum; then