Split tool from content: eec-campaigns becomes a standalone CLI

Campaign content and listmonk credentials now live in separate
per-domain repos that install this tool at a pinned version, so a
client with access to one domain's content repo has no path to
another domain's secrets. Renames the module to the actual Gitea
host path so `go install` works without vanity-import redirects,
drops the four workflows that assumed campaigns/ content lived here,
adds real go-test CI, and rewrites the README for the tool's new
audience.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

main.go

@@ -11,8 +11,8 @@ import (
 	"os"
 	"strings"
 
-	"reground.org/eec-campaigns/internal/campaign"
-	"reground.org/eec-campaigns/internal/listmonk"
+	"gitea.reground.org/will/eec-campaigns/internal/campaign"
+	"gitea.reground.org/will/eec-campaigns/internal/listmonk"
 )
 
 func main() {