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

script/frontend-viewport-test

@@ -31,8 +31,8 @@ HARNESS="netwatch-viewport-harness-$RUN_ID"
 ARTIFACT_DIR="$ROOT/tmp/viewport"
 
 # Every container is named and removed here, including the harness itself:
-# `timeout` below kills the `docker run` client, not the container it
-# started, and an unnamed survivor keeps the --internal network in use so
+# a `docker run` client that is killed outright leaves its container
+# running, and an unnamed survivor keeps the --internal network in use so
 # `docker network rm` fails too. This host runs many sessions at once and
 # neither may be left behind.
 cleanup() {
@@ -122,7 +122,7 @@ main() {
         -f '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' \
         "$BROWSER")"
 
-    timeout 900 docker run --rm --init --name "$HARNESS" \
+    docker run --rm --init --name "$HARNESS" \
         --network "$NETWORK" \
         --user "$(id -u):$(id -g)" \
         -v "$ROOT:/app" \