Initial implementation of the eec-campaigns tool

Drives listmonk's real Campaign API from git-authored Markdown+frontmatter,
so broadcast/segment emails get listmonk's mature unsubscribe/bulk-send/
attachment handling instead of reimplementing it. sync only ever creates or
updates a draft (idempotent, diff-based, refuses to touch a non-draft
campaign); a pushed send/<slug> tag or manual workflow run is the only way
to actually trigger a send. Includes list-name resolution, segment_query
materialization into managed lists, content-hash-deduped attachment
uploads, and an automatic post-sync preview email.

internal/campaign/send.go

@@ -0,0 +1,43 @@
+package campaign
+
+import (
+	"fmt"
+
+	"reground.org/eec-campaigns/internal/listmonk"
+)
+
+// Send transitions a campaign from draft/paused to running — the one real
+// send trigger in this whole tool (see cmd/send, triggered only by a pushed
+// send/<slug> git tag or a manual workflow_dispatch, never by a plain
+// sync). The campaign is looked up fresh by name each call, so this always
+// acts on listmonk's current live state, not whatever sync last saw.
+func Send(lm *listmonk.Client, slug string) (*listmonk.Campaign, error) {
+	camp, err := lm.FindCampaignByName(slug)
+	if err != nil {
+		return nil, err
+	}
+	if camp == nil {
+		return nil, fmt.Errorf("no campaign named %q in listmonk — run sync first", slug)
+	}
+	if camp.Status != "draft" && camp.Status != "paused" {
+		return nil, fmt.Errorf("campaign %q is %s in listmonk — only draft or paused campaigns can be sent", slug, camp.Status)
+	}
+	if err := lm.SetCampaignStatus(camp.ID, "running"); err != nil {
+		return nil, err
+	}
+	return camp, nil
+}
+
+// Test sends a preview of a campaign's current content to the given
+// addresses without touching its status — the same mechanism sync's
+// automatic preview uses, exposed standalone for on-demand re-previews.
+func Test(lm *listmonk.Client, slug string, emails []string) error {
+	camp, err := lm.FindCampaignByName(slug)
+	if err != nil {
+		return err
+	}
+	if camp == nil {
+		return fmt.Errorf("no campaign named %q in listmonk — run sync first", slug)
+	}
+	return lm.TestCampaign(camp.ID, emails)
+}