Fetch history and tags in the canonical workflow (closes #110)

The policy said a repository whose version comes from git tags needs `fetch-depth: 0` on its CI checkout, because the checkout action clones shallow with no tags; every repository's version comes from `git describe --tags --always`, but the canonical `.gitea/workflows/check.yml` did not set it, so CI stamped a bare short commit id where a local build of a tagged repository stamps the tag. The checkout step now sets `fetch-depth: 0` with a one-line comment, and the workflow bullet of `prompts/REPO_POLICIES.md` and both checklists name it beside `persist-credentials: false` and the `concurrency` block, as part of what the workflow does.

Unverified: a live run, which waits on the shared runner.

Model: opus-5-5

prompts/REPO_POLICIES.md

@@ -292,7 +292,10 @@ style conventions are in separate documents:
   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
+  later step. The checkout step also sets `fetch-depth: 0`, which fetches the
+  tags `git describe` needs: by default it clones shallow with no tags, and a
+  tagged repository's CI build would stamp a bare short commit id. The
+  workflow's `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.
@@ -472,8 +475,8 @@ style conventions are in separate documents:
     there because `ARG` is stage-scoped; passing `VERSION` to a repo whose
     Dockerfile declares no such `ARG` is ignored and costs nothing, which is why
     the scripts stay byte-identical. One consequence for CI: the standard
-    checkout action clones shallow and fetches no tags, so a repo that embeds a
-    tag-derived version must set `fetch-depth: 0` on its checkout step.
+    checkout action clones shallow and fetches no tags, so the canonical
+    `.gitea/workflows/check.yml` sets `fetch-depth: 0` on its checkout step.
 
 - **Verify `.dockerignore` by enumerating the image, not by reading the
   patterns.** Plant files at the root _and_ at least two directories deep, build