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

.gitea/workflows/check.yml

@@ -1,9 +1,16 @@
 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
         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
             - run: script/cibuild

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 `.gitignore` and `.editorconfig` now each end with a
   comment saying the repository's own entries go below it and a re-vendor keeps
   them (issue 103, which took in issue 104), as `.dockerignore`'s header already

prompts/EXISTING_REPO_CHECKLIST.md

@@ -96,7 +96,9 @@ with your task.
       `fetch-depth: 0` on the CI checkout step, which clones shallow and fetches
       no tags by default.
 - [ ] Gitea Actions workflow in `.gitea/workflows/` runs `script/cibuild` on
-      push — reference
+      push, checks out with `persist-credentials: false`, and carries the
+      `concurrency` block that lets a new push cancel only the same branch's
+      older run — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`
 - [ ] Language-specific config:
     - [ ] Go: `go.mod`, `go.sum`, `.golangci.yml` (fetch from

prompts/NEW_REPO_CHECKLIST.md

@@ -106,7 +106,9 @@ Template files can be fetched from:
     - Non-server: the final stage brings up the dev environment
     - Image pinned by sha256 hash with version/date comment
 - [ ] Gitea Actions workflow at `.gitea/workflows/check.yml` that runs
-      `script/cibuild` on push — reference
+      `script/cibuild` on push, checks out with `persist-credentials: false`,
+      and carries the `concurrency` block that lets a new push cancel only the
+      same branch's older run — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`
 - [ ] Language-specific:
     - [ ] Go: `go mod init sneak.berlin/go/<name>`, `.golangci.yml` (fetch from

prompts/REPO_POLICIES.md

@@ -283,9 +283,15 @@ style conventions are in separate documents:
       refuses an empty build argument drops that refusal and keeps the argument.
 
 - Every repo should have a Gitea Actions workflow (`.gitea/workflows/`) that
-  runs `script/cibuild` on push, and checks out the repo as its only other step.
-  That script bootstraps, runs the gate phases, and then builds the image, so a
-  successful run means every check passed; a bare `docker build .` does not
+  runs `script/cibuild` on push, and checks out the repo as its only other step,
+  with `persist-credentials: false`: `script/cibuild` needs no token, and
+  without it the checkout leaves the job's token in `.git/config` for every
+  later step. Its `concurrency` block groups runs by workflow and branch
+  (`${{ github.workflow }}-${{ github.ref }}`) with `cancel-in-progress: true`,
+  so a new push cancels the older run on the same branch, queued or running, and
+  no other: runs for replaced commits do not hold up the shared runner.
+  `script/cibuild` bootstraps, runs the gate phases, and then builds the image,
+  so a successful run means every check passed; a bare `docker build .` does not
   carry the same guarantee, because its gate phases may come from the cache. The
   image build is uncached and so runs the gate phases a second time. That is the
   price of the rule above, and it is worth paying: the image that ships is built