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

The canonical `.gitea/workflows/check.yml` lacked two settings `dnswatcher` had added, so a byte-identical re-vendor removed them. A `concurrency` block grouped by workflow and branch, with `cancel-in-progress: true`, makes a new push cancel the older run on the same branch and leaves every other branch's runs alone; on 2026-10-02 45 stale runs had queued on the one shared runner. `persist-credentials: false` on the checkout step keeps the job's token out of `.git/config`; `script/cibuild` needs no token. Each has a one-line comment, and the policy's workflow bullet and both checklists describe the file as it now is.

Unverified: the two live checks, which wait on the shared runner.

Model: opus-5-5

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