Re-vendor the canonical files from sneak/prompts at dd4027b (closes #13)

`.dockerignore`, `.gitignore`, `.golangci.yml` and the workflow are their
copies at that commit. This repo's own entries are kept after the
canonical content: the anchored `/attrsum` binary, the Go entries in
`.gitignore`, and tabs for `*.go` in the new `.editorconfig`.
`REPO_POLICIES.md` is new. The workflow keeps `fetch-depth: 0`, which the
policies require of a repo that stamps a tag-derived version.

The lint phase moves to golangci-lint v2.14.0, which finds nothing in
the code. `script/fmt` now runs goimports with `go run` at its pinned
commit. A version-checked install by `script/bootstrap` would have to be
found on `PATH`, and the CI runner's `PATH` lacks Go's bin directory.

Model: opus-5-5

.gitea/workflows/check.yml

@@ -4,7 +4,7 @@ jobs:
     check:
         runs-on: ubuntu-latest
         steps:
-            # actions/checkout v4.2.2, 2026-02-28
+            # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
               # Full history and tags, so the build stamps the same
               # `git describe` version as a full clone.