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/store/topic_store.go

@@ -23,17 +23,15 @@ type TopicStore struct {
 func NewTopicStore(db *sqlx.DB) *TopicStore { return &TopicStore{db: db} }
 
 type CreateTopicInput struct {
-	Title                string
-	Summary              string
-	Visibility           models.Visibility
-	VerdictSchemaID      string
-	SignupOpenAt         time.Time
-	SignupCloseAt        time.Time
-	DebateStartAt        time.Time
-	DebateEndAt          time.Time
-	CreatorUserID        string
-	AnnounceGuildBaseURL *string // optional; null = no broadcasts for this topic
-	AnnounceChannelID    *string
+	Title           string
+	Summary         string
+	Visibility      models.Visibility
+	VerdictSchemaID string
+	SignupOpenAt    time.Time
+	SignupCloseAt   time.Time
+	DebateStartAt   time.Time
+	DebateEndAt     time.Time
+	CreatorUserID   string
 }
 
 func (s *TopicStore) Create(ctx context.Context, in CreateTopicInput) (*models.Topic, error) {
@@ -41,11 +39,11 @@ func (s *TopicStore) Create(ctx context.Context, in CreateTopicInput) (*models.T
 	_, err := s.db.ExecContext(ctx, `
 		INSERT INTO topics (id, title, summary, visibility, verdict_schema_id,
 			signup_open_at, signup_close_at, debate_start_at, debate_end_at,
-			creator_user_id, announce_guild_base_url, announce_channel_id)
-		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+			creator_user_id)
+		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
 		id, in.Title, in.Summary, in.Visibility, in.VerdictSchemaID,
 		in.SignupOpenAt, in.SignupCloseAt, in.DebateStartAt, in.DebateEndAt,
-		in.CreatorUserID, in.AnnounceGuildBaseURL, in.AnnounceChannelID)
+		in.CreatorUserID)
 	if err != nil {
 		return nil, fmt.Errorf("insert topic: %w", err)
 	}