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

.gitea/workflows/check.yml

@@ -7,8 +7,6 @@ concurrency:
 jobs:
     check:
         runs-on: ubuntu-latest
-        # Free the shared runner from a hung build.
-        timeout-minutes: 20
         steps:
             # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
@@ -17,4 +15,7 @@ jobs:
                   persist-credentials: false
                   # All history and tags, so git describe finds the version tag.
                   fetch-depth: 0
+            # Free the shared runner from a hung build. The limit is on the
+            # step because the runner ignores a job's timeout-minutes.
             - run: script/cibuild
+              timeout-minutes: 20