Phase 0: project scaffold and build pipeline

Go module (codeberg.org/mix/selfpost), two command skeletons (panel,
selfpost-backup) sharing internal/buildinfo for the -ldflags version stamp,
Makefile (static CGO_ENABLED=0 build), AGPL-3.0 LICENSE, README skeleton and
.gitattributes forcing LF (container scripts must not get CRLF).

Verified on the dev server: go vet clean, make build produces statically
linked binaries, version stamping works.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

cmd/selfpost-backup/main.go

@@ -0,0 +1,26 @@
+// Command selfpost-backup produces (and helps restore) the full persistent-state
+// archive from inside the container, invoked via `docker exec` for scripted/cron
+// backups — the CLI equivalent of the panel's backup button (spec 7.5.A, 11.6).
+//
+// This is the Phase 0 skeleton: it only reports its version. The actual archive
+// logic lands in Phase 9.
+package main
+
+import (
+	"flag"
+	"fmt"
+
+	"codeberg.org/mix/selfpost/internal/buildinfo"
+)
+
+func main() {
+	showVersion := flag.Bool("version", false, "print version and exit")
+	flag.Parse()
+
+	if *showVersion {
+		fmt.Println(buildinfo.Version)
+		return
+	}
+
+	fmt.Printf("selfpost-backup %s (skeleton)\n", buildinfo.Version)
+}