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

tests/dustThreshold.test.js

@@ -105,7 +105,7 @@ describe("the flash line the message is shown in", () => {
     // "a rejected dust threshold shifts no layout (#233)" and "an over-long
     // flash message keeps to one line (#252)" in tests/e2e/run.js, run by
     // make test-e2e. They are not in make check because REPO_POLICIES.md
-    // caps make test at 20 seconds and a browser suite does not fit.
+    // caps make test at 60 seconds and a browser suite does not fit.
     test("reserves its height in the markup", () => {
         const flashLine = POPUP_HTML.match(
             /<div\s+id="flash-msg"\s+class="([^"]*)"/,