feat: lifecycle broadcasts on signup_closed / cancelled / debating / completed

Phase 3 push-wakeup mechanism without adding a new push channel.
Topic state transitions now post short messages to the same Fabric
announce channel used for the initial signup announcement. Agents
subscribed to announce + not currently busy get woken via the
existing Phase 1 inbound path; busy-discard already filters
appropriately. No SSE, no per-agent DM fanout, no plugin changes —
reuses existing infra end-to-end.

Changes:
- ticker.go: after signup_close transition, broadcasts signup_closed
  (with pro/con/judge agent IDs + debate-start time) OR cancelled
  (with reason). After debate_start transition, broadcasts debating
  with debate-end time.
- announce.go: new PostLifecycleEvent helper - same headers/auth as
  PostTopicAnnouncement, different format.
- verdict.go: after successful judge submission, broadcasts completed
  with the judge id. Best-effort + async so a slow Fabric does not
  slow the judge response.
- routes.go: instantiates the announcer once + passes to VerdictHandler.

Workflow participate-debate step 5 should be updated to expect
wakeups instead of polling - separate follow-up edit on lyn/ClawSkills.

internal/fabric/announce.go

@@ -115,6 +115,47 @@ func formatAnnouncement(id, title, summary string,
 	)
 }
 
+// PostLifecycleEvent broadcasts a non-signup state change to the
+// announce channel. The agents' Fabric inbound + busy-discard
+// (Phase 1) handles per-agent wakeup gating — no per-agent fanout
+// needed from this side. Sole purpose is to let agents see "your
+// debate moved to X" without polling.
+//
+// Currently called for: signup_closed (with allocated camps),
+// cancelled, debating-start, completed. Format is plain text with the
+// topic_id so agents' workflows can parse and route.
+func (a *Announcer) PostLifecycleEvent(ctx context.Context, topicID, title, kind, summary string) error {
+	if !a.Enabled() {
+		log.Printf("announce: lifecycle skipped (fabric coupling not configured) topic=%s kind=%s", topicID, kind)
+		return nil
+	}
+	body := map[string]any{
+		"content": fmt.Sprintf("📣 **[%s]** %s [%s]\n%s", kind, title, topicID, summary),
+	}
+	raw, _ := json.Marshal(body)
+	url := fmt.Sprintf("%s/api/channels/%s/messages",
+		trimRightSlash(a.cfg.GuildBaseURL), a.cfg.ChannelID)
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, url, bytes.NewReader(raw))
+	if err != nil {
+		return err
+	}
+	req.Header.Set("content-type", "application/json")
+	req.Header.Set("authorization", "Bearer "+a.cfg.BotBearerToken)
+	req.Header.Set("x-fabric-system-key", a.cfg.SystemAPIKey)
+	resp, err := a.client.Do(req)
+	if err != nil {
+		log.Printf("announce: lifecycle POST %s failed: %v", url, err)
+		return err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode >= 300 {
+		b, _ := io.ReadAll(io.LimitReader(resp.Body, 1024))
+		log.Printf("announce: lifecycle POST %s -> %d body=%s", url, resp.StatusCode, string(b))
+		return fmt.Errorf("lifecycle post: status %d", resp.StatusCode)
+	}
+	return nil
+}
+
 func trimRightSlash(s string) string {
 	for len(s) > 0 && s[len(s)-1] == '/' {
 		s = s[:len(s)-1]