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>

.gitea/workflows/ci.yml

@@ -0,0 +1,21 @@
+name: CI
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-go@v5
+        with:
+          go-version: "1.23.1"
+
+      - name: Build
+        run: go build ./...
+
+      - name: Test
+        run: go test ./...