Cancel replaced CI runs and drop the checkout token (closes #107)

The canonical `.gitea/workflows/check.yml` gains a `concurrency` block grouped
by workflow and branch with `cancel-in-progress: true`, so a new push cancels
the older run on the same branch and no other, and its checkout step sets
`persist-credentials: false`, so the job's token is not left in `.git/config`;
`script/cibuild` needs none. Both come from `dnswatcher`, where a byte-identical
re-vendor would have removed them. The workflow bullet of
`prompts/REPO_POLICIES.md` and both checklists now describe the file as it is.

Model: opus-5-5

TODO.md

@@ -21,6 +21,12 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-06: The canonical `.gitea/workflows/check.yml` now has a `concurrency`
+  block, so a new push cancels the older run on the same branch and no other,
+  and its checkout step sets `persist-credentials: false`, so the job's token is
+  not left in `.git/config` (issue 107). `REPO_POLICIES.md` and both checklists
+  describe the workflow as it now is. Not yet tried on the shared runner, which
+  is out of disk space. Repositories pick this up on their next re-vendor.
 - 2026-10-06: The canonical `.golangci.yml` now disables `canonicalheader`
   (issue 105). In golangci-lint v2.14.0 it misses findings at random in a
   package that also calls `ResponseWriter.Header()`, so the same tree can fail