Adopt the scripts-to-rule-them-all layout and the canonical Docker gates

script/ holds the entrypoints and the Makefile targets shim to them.
Linting and testing are phases of the Dockerfile, built alone by
script/lint and script/test with a fresh CACHEBUST value and cache
mounts for Go's build cache and golangci-lint's; the build stage
depends on both, and the runtime stage ships the binary and the source
without .git. Formatting stays on the host in script/fmt and
script/fmt-check (gofmt and the prettier pinned in yarn.lock).
script/cibuild bootstraps, runs script/check and builds the tagged
image, and the Gitea workflow calls it. script/precommit refuses a
commit whose go.mod or go.sum is not tidy.

cgo stays off, so the test phase runs on the alpine Go image without
-race; that is the one departure from the canonical Go Dockerfile.

Re-vendored from the prompts repository's next branch: REPO_POLICIES.md
(2026-11-04), .golangci.yml (byte-identical, so the local
exhaustruct_v5 addition is gone now that canonical carries it),
.gitea/workflows/check.yml, .editorconfig, .gitignore and .dockerignore,
the last two followed by this repository's own entries. AGENTS.md holds
the repository-specific guidance for agents. The README gains the
Entrypoints section and describes the new gates.

script/lint

@@ -0,0 +1,28 @@
+#!/bin/sh
+# script/lint: run the linter. Linting is a phase of the Dockerfile and
+# this builds that phase alone; the linter is never installed or run on
+# a developer host, where a shared result cache and a host-global lock
+# make its answer untrustworthy.
+#
+# The phase is not the last stage in the file, so it is built only when
+# --target names it. A new CACHEBUST value reruns every RUN step after
+# the ARG CACHEBUST line, since a cached lint layer is a lint that did
+# not run; unlike --no-cache, it keeps the layers above that line and
+# the cache mounts. This relies on every stage of the Dockerfile
+# declaring ARG CACHEBUST before every step that lints, tests or
+# compiles; a stage without that line is served from cache on an
+# unchanged tree, with no warning.
+# --output type=cacheonly writes no image, since nothing uses one.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
+
+main() {
+    cd "$ROOT"
+    docker build --build-arg CACHEBUST="$(date +%s)-$$" \
+        --target lint \
+        --output type=cacheonly .
+}
+
+main "$@"