CI checkout fetches all history and tags (closes #153)

The checkout step cloned one commit and fetched no tags, so git
describe in script/cibuild gave the image built from a tagged commit
the short commit as its version. .gitea/workflows/check.yml is now the
sneak/prompts copy at 0653579, whose checkout step sets fetch-depth: 0.
That copy also sets persist-credentials: false on the checkout, cancels
the older run on the same branch when a new push arrives, and stops a
run after 20 minutes. The TODO.md status line says the workflow cancels
the older run.

Model: opus-5-5

.gitea/workflows/check.yml

@@ -1,9 +1,20 @@
 name: check
 on: [push]
+# Free the shared runner: a new push cancels only the same branch's older run.
+concurrency:
+    group: ${{ github.workflow }}-${{ github.ref }}
+    cancel-in-progress: true
 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
+              # script/cibuild needs no token, so none is left in .git/config.
+              with:
+                  persist-credentials: false
+                  # All history and tags, so git describe finds the version tag.
+                  fetch-depth: 0
             - run: script/cibuild