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>

internal/buildinfo/buildinfo.go

@@ -0,0 +1,10 @@
+// Package buildinfo exposes build-time metadata stamped via -ldflags.
+package buildinfo
+
+// Version is the SelfPost release. It is set at build time with
+//
+//	-ldflags "-X codeberg.org/mix/selfpost/internal/buildinfo.Version=<tag>"
+//
+// and must match the Docker image tag; it is used for the backup/restore
+// compatibility check (spec 7.5.A). Defaults to "dev" for local/unstamped builds.
+var Version = "dev"