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, which
now fetches apt package lists so a fresh CI runner can install Go. The
image takes its version from the VERSION build arg or git describe, dev
without .git, and still stamps the commit and its date. This repo's own
entries follow the canonical content in .gitignore and .editorconfig.
The golangci-lint v2.14.0 findings are fixed in the code. The rules in
CLAUDE.md move into AGENTS.md. IsDevVersion now counts "unknown".

Model: opus-5-5

script/install-go

@@ -7,9 +7,11 @@
 # Only .gitea/workflows/release.yml calls this. goreleaser is not a
 # compiler: it shells out to `go` for the `before:` hook and for every
 # one of the four cross-compiles, so the release runner needs a Go
-# toolchain on PATH. check.yml never does -- it builds inside the
-# digest-pinned Dockerfile images -- so this is the release path's only
-# host Go, and per REPO_POLICIES.md it must be pinned by hash.
+# toolchain on PATH. check.yml compiles nothing on the host: it uses
+# whatever Go script/bootstrap finds or installs only for that script's
+# `go mod download` and for gofmt in script/fmt-check. So this is the
+# only host Go that builds anything, and per REPO_POLICIES.md it must be
+# pinned by hash.
 # actions/setup-go exposes no checksum input, so Go is installed the way
 # script/install-goreleaser installs goreleaser: download the exact
 # archive from go.dev and refuse it unless its sha256 matches the value
@@ -18,7 +20,8 @@
 # The version is go.mod's `go` directive, the single source of truth for
 # the toolchain. GO_VERSION below MUST equal it, and this script fails
 # when they disagree -- so bumping Go is one reviewed change touching
-# go.mod, the checksum here, and the Dockerfile golang digest together.
+# go.mod, the checksum here, and the Dockerfile's two golang digests
+# together.
 #
 # Linux only, because that is what the release runner is. A darwin dev
 # building a snapshot uses their own Go; supporting an OS means adding