Set a time limit on the canonical check job (closes #120)

The canonical check workflow gave its job no time limit, so a hung
script/cibuild held the shared runner until the runner's own default.
The check job now sets timeout-minutes: 20. script/cibuild runs three
Docker builds (the test phase, the lint phase, then the image, which
runs both again), each held to the 5-minute build limit, plus the
bootstrap. REPO_POLICIES.md and both checklists name the limit among
what the workflow sets.

Model: opus-5-5

.gitea/workflows/check.yml

@@ -7,6 +7,8 @@ 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