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.

.editorconfig

@@ -10,3 +10,9 @@ insert_final_newline = true
 
 [Makefile]
 indent_style = tab
+
+[*.go]
+indent_style = tab
+
+# This repository's own sections, such as one for another language it
+# uses, go below this comment, and a re-vendor keeps them.