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

script/install-precommit

@@ -1,19 +1,15 @@
 #!/bin/sh
 # script/install-precommit: install the git pre-commit hook that runs
 # script/precommit. Our own extension to scripts-to-rule-them-all.
-# Hooks are shared between the main checkout and all worktrees, so
-# resolve the common git dir instead of assuming .git is a directory.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    hooks_dir="$(git rev-parse --git-common-dir)/hooks"
-    mkdir -p "$hooks_dir"
-    hook="$hooks_dir/pre-commit"
-    printf '#!/bin/sh\nset -e\nscript/precommit\n' > "$hook"
-    chmod +x "$hook"
+    hook=".git/hooks/pre-commit"
+    printf '#!/bin/sh\nset -e\nscript/precommit\n' > .git/hooks/pre-commit
+    chmod +x .git/hooks/pre-commit
     echo "pre-commit hook installed: runs script/precommit"
 }