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/EXISTING_REPO_CHECKLIST.md

@@ -96,13 +96,15 @@ with your task.
       directory outside the build context, so the build cannot read the version
       and a plain `docker build .` fails; pass the version with
       `--build-arg VERSION=...`. `script/docker` and `script/cibuild` already
-      pass the version they compute on the host; it takes precedence. A
-      tag-derived version additionally needs `fetch-depth: 0` on the CI checkout
-      step, which clones shallow and fetches no tags by default.
+      pass the version they compute on the host; it takes precedence. The
+      canonical `.gitea/workflows/check.yml` sets `fetch-depth: 0` on its
+      checkout step, which otherwise clones shallow and fetches no tags, so a CI
+      build finds the tag too.
 - [ ] Gitea Actions workflow in `.gitea/workflows/` runs `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
+      push, checks out with `persist-credentials: false` and with
+      `fetch-depth: 0` (which fetches the tags `git describe` needs), 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