Re-vendor the canonical files from sneak/prompts dd4027b (closes #159, closes #116)

Fetches .dockerignore, .editorconfig, the CI workflow, .gitignore,
.golangci.yml, .prettierignore, .prettierrc and REPO_POLICIES.md byte
for byte from sneak/prompts dd4027b; this repo's own entries follow the
canonical text in .dockerignore, .gitignore and .editorconfig. The new
.golangci.yml disables gomodguard. The Dockerfile gets a lint phase on
golangci-lint v2.14.0 and a test phase on the Debian Go image; the
build stage depends on both and stamps the version as the policy shows.
script/test and script/lint build only their phase, and script/cibuild
bootstraps and runs script/check first. bin/tools goes: script/bootstrap
installs gofumpt and protoc-gen-go into bin/ with go install pinned to a
commit.

Model: opus-5-5

.gitea/workflows/check.yml

@@ -4,6 +4,6 @@ jobs:
     check:
         runs-on: ubuntu-latest
         steps:
-            # actions/checkout v4.2.2, 2026-03-16
+            # actions/checkout v4.2.2, 2026-02-22
             - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
             - run: script/cibuild