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-test

@@ -15,13 +15,13 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 main() {
     cd "$ROOT"
     NODE_OPTIONS=--test-reporter=dot \
-        timeout 90 "$ROOT/script/yarn" --silent run test || {
+        "$ROOT/script/yarn" --silent run test || {
         echo "--- Rerunning with every test listed for details ---"
         NODE_OPTIONS=--test-reporter=spec \
-            timeout 90 "$ROOT/script/yarn" --silent run test
+            "$ROOT/script/yarn" --silent run test
         exit 1
     }
-    timeout 30 "$ROOT/script/yarn" build
+    "$ROOT/script/yarn" build
 }
 
 main "$@"