refactor(backend): drop backend-driven Fabric broadcast — agent-driven model

The backend no longer broadcasts topic lifecycle events to Fabric. The
new model (per design discussion 2026-05-23 evening):

  - Proposing agent posts a single recruitment fabric-send-message
    immediately after creating a topic (carries topic_id + signup
    window + debate window + title).
  - Downstream agents that decide to participate book a HF on_call
    slot covering the debate window via `hf calendar schedule on_call
    <time> <duration> --job DEBATE-<topic_id>`.
  - HF wakes the agent naturally at slot start; the wake payload
    carries event_data with the DEBATE-<topic_id> code so the agent
    knows why it was woken.
  - The backend stays a pure data + state-machine service and doesn't
    know about Fabric.

Code removed:

  - internal/fabric/announce.go (entire file + empty dir)
  - ticker.go: broadcastLifecycle + broadcastAnnouncement + topicTarget
    helpers; announcer field on Ticker; announce field/arg on NewTicker
  - models/topic.go: AnnounceGuildBaseURL + AnnounceChannelID fields
  - store/topic_store.go: same fields on CreateTopicInput + INSERT
  - handlers/topics.go: same fields on createTopicBody + validation +
    parameter passing to store
  - handlers/verdict.go: announcer field + lifecycle broadcast on
    verdict submit
  - config/config.go: FabricSystemAPIKey field + DIALECTIC_FABRIC_SYSTEM_API_KEY
    env read
  - main.go + routes.go: announcer wiring

Database:

  - migrations/003_drop_topic_announce_target.sql drops the two columns
    added by migration 002. Counterpart commit on the deployment side
    needs DIALECTIC_FABRIC_SYSTEM_API_KEY env removed from
    docker-compose.yml; harmless if left as the backend no longer
    reads it.

Pairs with:
  - Dialectic.OpenclawPlugin: rip announce_* params from
    dialectic_propose_topic (next commit)
  - Fabric.Backend.Center: rip serviceEndpoint field + cli
  - Fabric.Backend.Guild: rip system-key bypass on ApiKeyGuard and
    announce-only-system limit on messaging.controller
  - ClawSkills: rewrite participate-debate + analyze-intel step 4 +
    delete rotate-fabric-system-key workflow

internal/httpapi/handlers/topics.go

@@ -109,12 +109,6 @@ type createTopicBody struct {
 	SignupCloseAt   string `json:"signup_close_at"`
 	DebateStartAt   string `json:"debate_start_at"`
 	DebateEndAt     string `json:"debate_end_at"`
-	// Optional: per-topic announce-channel target. Both must be set
-	// (or both omitted = no broadcasts). Creator picks based on the
-	// debate's intended audience (different guilds may host different
-	// communities, different channels may serve different categories).
-	AnnounceGuildBaseURL string `json:"announce_guild_base_url,omitempty"`
-	AnnounceChannelID    string `json:"announce_channel_id,omitempty"`
 }
 
 // POST /api/topics
@@ -147,31 +141,16 @@ func (h *TopicsHandler) Create(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}
-	// Announce target: both fields required or both empty; one-of-two
-	// is a config error caught here rather than silently treated as
-	// "no broadcast".
-	var aGuild, aChannel *string
-	if body.AnnounceGuildBaseURL != "" || body.AnnounceChannelID != "" {
-		if body.AnnounceGuildBaseURL == "" || body.AnnounceChannelID == "" {
-			http.Error(w, "announce_guild_base_url and announce_channel_id must both be set (or both empty for no broadcasts)", http.StatusBadRequest)
-			return
-		}
-		g, c := body.AnnounceGuildBaseURL, body.AnnounceChannelID
-		aGuild, aChannel = &g, &c
-	}
-
 	created, err := h.store.Create(r.Context(), store.CreateTopicInput{
-		Title:                body.Title,
-		Summary:              body.Summary,
-		Visibility:           models.Visibility(body.Visibility),
-		VerdictSchemaID:      body.VerdictSchemaID,
-		SignupOpenAt:         parsed[0],
-		SignupCloseAt:        parsed[1],
-		DebateStartAt:        parsed[2],
-		DebateEndAt:          parsed[3],
-		CreatorUserID:        caller.ID,
-		AnnounceGuildBaseURL: aGuild,
-		AnnounceChannelID:    aChannel,
+		Title:           body.Title,
+		Summary:         body.Summary,
+		Visibility:      models.Visibility(body.Visibility),
+		VerdictSchemaID: body.VerdictSchemaID,
+		SignupOpenAt:    parsed[0],
+		SignupCloseAt:   parsed[1],
+		DebateStartAt:   parsed[2],
+		DebateEndAt:     parsed[3],
+		CreatorUserID:   caller.ID,
 	})
 	if err != nil {
 		http.Error(w, "create failed: "+err.Error(), http.StatusInternalServerError)