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

.dockerignore

@@ -17,7 +17,17 @@
 # stage that compiles runs `git describe --tags --always` on .git, which
 # does not need .git/config; that file can hold a credential, such as a
 # password in a remote URL or the token the CI checkout step stores there.
-.git/config
+# Each submodule keeps a config with the same exposure in its git directory
+# under .git/modules/, nested again for a submodule's own submodules, or in
+# its own .git directory when it keeps one.
+# KNOWN GAP: a submodule whose name has a `config` segment (`config`,
+# `deploy/config`, `config/lib`) loses its whole git directory, because
+# `**/.git/modules/**/config` also matches that segment's directory
+# under .git/modules/. Go's version stamping then fails the build;
+# nothing leaks. Name such a submodule without that segment:
+# `git submodule add --name`.
+**/.git/config
+**/.git/modules/**/config
 
 # Agent scratch: one full checkout of the repo per in-flight agent.
 # Anchored because it occurs once where agents run at the repo root.
@@ -41,7 +51,9 @@
 **/[iI][dD]_[rR][sS][aA]
 **/[iI][dD]_[dD][sS][aA]
 **/[iI][dD]_[eE][cC][dD][sS][aA]
+**/[iI][dD]_[eE][cC][dD][sS][aA]_[sS][kK]
 **/[iI][dD]_[eE][dD]25519
+**/[iI][dD]_[eE][dD]25519_[sS][kK]
 
 # Dependencies: restored inside the image, never copied in.
 **/node_modules