Bring the repo up to the standard layout (closes #1)

Adds the canonical .gitignore, .dockerignore and .editorconfig; the
first two also name this repo's build output, root-anchored so
cmd/bsdaily/ stays in. The Dockerfile gains a lint phase on the
golangci-lint image already pinned and a test phase on the Debian Go
image with sqlite3 and zstd; the build stage copies a file from each,
so a plain docker build cannot skip them. script/lint and script/test
build their phase uncached and tagged; script/cibuild, script/docker
and the CI workflow are the canonical copies. Nothing installs
golangci-lint on the host. REPO_POLICIES.md is re-vendored.

.golangci.yml and the lint cleanup: #6
Judgement call: .gitignore adds Go build output, per the Go styleguide.

Model: opus-5-5

script/lint

@@ -1,12 +1,23 @@
 #!/bin/sh
-# script/lint: run the linter.
+# 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. --no-cache because a cached lint layer is a lint
+# that did not run. The tag makes each build replace the previous image
+# instead of leaving a dangling one behind.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    golangci-lint run ./...
+    docker build --no-cache \
+        --target lint \
+        -t "$("$SCRIPT_DIR/projectname")-lint" .
 }
 
 main "$@"