Add scripts-to-rule-them-all scaffold (refs #1)

Add the standard STRTA scaffold, mirroring the conformant Go repos:

- script/ POSIX-sh entrypoints (bootstrap, setup, projectname, test,
  lint, fmt, fmt-check, check, docker, cibuild, precommit,
  install-precommit).
- Makefile rewritten as thin shims: .PHONY plus the nine standard
  targets each delegating to script/NAME; repo-specific build, clean,
  and try targets retained.
- .golangci.yml matching the org-standard Go lint config.
- Dockerfile whose build runs make check then make build, so the image
  fails on any check failure.
- .gitea/workflows/check.yml running script/cibuild.

make fmt-check and make test are green. make check is not yet green
because of pre-existing lint findings in the application code, which
are out of scope for this scaffold change; hence refs (not closes).

script/check

@@ -0,0 +1,14 @@
+#!/bin/sh
+# script/check: run all checks (test, lint, fmt-check). Our own
+# extension to scripts-to-rule-them-all. Must not modify any files.
+set -eu
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+
+main() {
+    "$SCRIPT_DIR/test"
+    "$SCRIPT_DIR/lint"
+    "$SCRIPT_DIR/fmt-check"
+}
+
+main "$@"