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. `script/bootstrap`
installs goimports with `go install` at its pinned commit unless the
installed binary already has the pinned version, read with
`go version -m`. It and `script/fmt` put Go's bin directory on `PATH`
first, since the CI runner's `PATH` lacks it.

Model: opus-5-5

script/bootstrap

@@ -4,14 +4,16 @@
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes nothing is present.
 # goimports is installed via `go install` at a pinned commit (never
-# "latest"). The linter is not installed: it runs only as the lint phase
-# of the Dockerfile.
+# "latest"), unless the installed one already has the pinned version.
+# The linter is not installed: it runs only as the lint phase of the
+# Dockerfile.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 # Pinned versions, 2026-10-05
-# goimports v0.42.0
+# GOIMPORTS_REF is the commit tagged GOIMPORTS_VERSION.
+GOIMPORTS_VERSION="v0.42.0"
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
 
 PKGMGR=""
@@ -61,6 +63,30 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
+# Print the version the goimports on PATH was built from, or nothing.
+# goimports has no version flag; `go version -m` reads the binary's
+# build info, whose "mod" line names the module and its version.
+goimports_version() {
+    if missing goimports; then return 0; fi
+    go version -m "$(command -v goimports)" 2>/dev/null |
+        awk '$1 == "mod" { print $3 }'
+}
+
+ensure_goimports() {
+    if [ "$(goimports_version)" = "$GOIMPORTS_VERSION" ]; then
+        echo "goimports $GOIMPORTS_VERSION already installed"
+        return 0
+    fi
+    go install "$GOIMPORTS_REF"
+    hash -r
+    if [ "$(goimports_version)" != "$GOIMPORTS_VERSION" ]; then
+        echo "bootstrap: goimports on PATH is not $GOIMPORTS_VERSION:" \
+            "$(command -v goimports)" >&2
+        exit 1
+    fi
+    echo "goimports $GOIMPORTS_VERSION installed"
+}
+
 main() {
     cd "$ROOT"
 
@@ -68,9 +94,11 @@ main() {
     if missing make; then pkg_install gnumake make make make; fi
     if missing go; then pkg_install go golang go go; fi
 
-    # Format tool, pinned via go install (installs into
-    # "$(go env GOPATH)/bin"; ensure that is on your PATH).
-    if missing goimports; then go install "$GOIMPORTS_REF"; fi
+    # go install writes to Go's bin directory, which need not be on PATH.
+    gobin="$(go env GOBIN)"
+    [ -n "$gobin" ] || gobin="$(go env GOPATH)/bin"
+    PATH="$gobin:$PATH"
+    ensure_goimports
 
     go mod download