CI time limit on the step; no timeout in Dockerfile or scripts (closes #199)

The runner ignores a job's `timeout-minutes`, so the 20-minute limit moves to the step that runs `script/cibuild`. The `Dockerfile`, `script/frontend-test` and `script/frontend-viewport-test` drop their `timeout` wrappers, which also stopped `make check` outside CI. `go test -timeout 90s` stays, as `REPO_POLICIES.md` requires it.

Deviation: `REPO_POLICIES.md`, vendored from `sneak/prompts`, still says the job sets the limit.

Model: opus-5-5

Dockerfile

@@ -62,9 +62,9 @@ WORKDIR /app
 COPY package.json yarn.lock ./
 RUN yarn install --frozen-lockfile
 COPY . .
-RUN NODE_OPTIONS=--test-reporter=dot timeout 90 yarn --silent run test || \
+RUN NODE_OPTIONS=--test-reporter=dot yarn --silent run test || \
     { echo "--- Rerunning with every test listed for details ---"; \
-      NODE_OPTIONS=--test-reporter=spec timeout 90 yarn --silent run test; \
+      NODE_OPTIONS=--test-reporter=spec yarn --silent run test; \
       exit 1; }
 
 # Test phase: the backend's tests with the race detector and coverage,