chore: re-vendor canonical files from prompts at dd4027b (closes #472)

Copies .dockerignore, .gitignore, .prettierignore, check.yml and
REPO_POLICIES.md from sneak/prompts at dd4027b. The repo's own entries
(dist/, release/, yarn files) are kept after the canonical content.

The Dockerfile gets separate lint and test phases. Its last stage
depends on both, checks the git describe version and runs make build.
script/lint, test, check, cibuild and docker are the canonical models.
check-censored moves into the lint phase and test-verify-build into the
test phase. fmt and fmt-check fall back to the nvm-installed node. The
e2e image builds are uncached. Comments that cited the old 20-second
test cap now say 60.

Model: opus-5-5

build.js

@@ -40,7 +40,7 @@ const AUDITED_MODULE = "src/shared/constants.js";
 // the build, whether or not a text matcher would have recognized it. A
 // background entry point the table does not name fails as well, so a second
 // worker is protected by default rather than by someone remembering this file.
-// Dockerfile:42 runs `make build`, so it is enforced in CI.
+// The Dockerfile's last stage runs `make build`, so it is enforced in CI.
 
 // The build receipt: every file this build emits, with its sha256 and whether
 // it is one of the audited bundles. script/verify-build is handed this and