build: re-vendor canonical files from prompts dd4027b (closes #257)

The canonical files are fetched whole from sneak/prompts at dd4027b.
Kept after the canonical content: the livednstest deny entry in
.golangci.yml, /bin in .dockerignore, this repo's own .gitignore lines
and a [*.go] tab section in .editorconfig. The workflow keeps its
concurrency block and persist-credentials: false. Lint and test are
phases of the Dockerfile (golangci-lint v2.14.0; tests on the Debian
Go 1.25.7 image as an ordinary user, with the same flags); the build
stage depends on both and stamps the version the canonical way.
Dockerfile.lint is gone; the prettier stages of Dockerfile.fmt moved
into the Dockerfile. Every scripted docker build passes --no-cache;
script/cibuild bootstraps, runs script/check, then builds the image.
script/fmt-check absorbs fmt-check-go and fmt-check-markdown.

Model: opus-5-5

.golangci.yml

@@ -17,6 +17,7 @@ linters:
   disable:
     # Genuinely incompatible with project patterns
     - exhaustruct      # Requires all struct fields
+    - exhaustruct_v5   # Requires all struct fields (successor to exhaustruct)
     - godot            # Requires comments to end with periods
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go