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, keeping the repo's own
entries (dist/, release/, yarn files) 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 citing the old test caps or what
runs a script are updated.

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