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

TODO.md

@@ -21,6 +21,14 @@ fmt-check, and commit.
 
 # Completed Steps
 
+- 2026-10-06: The canonical `.gitea/workflows/check.yml` now sets
+  `fetch-depth: 0` on its checkout step (issue 110), so CI fetches the history
+  and tags that `git describe --tags --always` needs, and a tagged repository
+  stamps the same version in CI as in a local build. `REPO_POLICIES.md` and both
+  checklists name `fetch-depth: 0` among what the workflow does, next to
+  `persist-credentials: false` and the `concurrency` block, instead of asking
+  each tagged repository to add it. 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 `script/bootstrap` now runs `apt-get update` once,
   before the first `apt-get install` of a run (issue 115). The Gitea runner
   image starts with empty package lists, so installing anything it lacks, such