Re-vendor the canonical files from sneak/prompts at dd4027b (closes #213)

Linting and testing become the lint and test phases of the Dockerfile,
and the build stage depends on both. Dockerfile.lint, CHECK_EPOCH and
the tests that checked them are removed. Every docker build in script/
passes --no-cache, and script/cibuild runs script/bootstrap first. A
host without Go gets the go.mod version from script/install-go in
.tool/go, where script/fmt and script/fmt-check also look; fmt-check
reads only the Go files git lists. The image takes its version from the
VERSION build arg or git describe, dev without .git. This repo's own
entries follow the canonical content in .gitignore and .editorconfig.
The golangci-lint v2.14.0 findings are fixed. The rules in CLAUDE.md
move into AGENTS.md. IsDevVersion counts "unknown".

Model: opus-5-5

.gitea/workflows/release.yml

@@ -16,11 +16,12 @@ jobs:
           fetch-depth: 0
       # goreleaser is not a compiler: it shells out to `go` for the
       # `before:` hook and for every one of the four cross-compiles.
-      # Nothing else in this repo puts a Go toolchain on the runner --
-      # check.yml runs script/cibuild, which does all of its work inside
-      # the digest-pinned Dockerfile images -- so without this step the
-      # release either fails at the before-hook or, worse, ships binaries
-      # built by whatever Go the runner happens to carry.
+      # Without this step the release either fails at the before-hook or,
+      # worse, ships binaries built by whatever Go the runner happens to
+      # carry. check.yml's runner gets the same Go through
+      # script/bootstrap, which calls script/install-go, and uses it only
+      # for `go mod download` and gofmt; it compiles inside the
+      # digest-pinned Dockerfile images.
       #
       # actions/setup-go would pin the action by commit sha, but the Go
       # tarball it downloads at runtime is verified against no value in