feat: Wire up created_by field for beads issues (gt-u6nri)

- Add CreatedBy field to Issue struct (matches beads GH#748)
- Add Actor field to CreateOptions, pass --actor to bd create
- Add ActorString() method to RoleInfo for identity formatting
- Update all beads.Create() callers to pass Actor
- Update direct bd create exec calls with --actor:
  - mail/router.go: uses sender identity
  - patrol_helpers.go: uses role name
  - doctor/patrol_check.go: uses "gt-doctor"
  - rig/manager.go: uses "gt-rig-init"

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mail/router.go

@@ -144,6 +144,9 @@ func (r *Router) Send(msg *Message) error {
 		args = append(args, "--labels", strings.Join(labels, ","))
 	}
 
+	// Add actor for attribution (sender identity)
+	args = append(args, "--actor", msg.From)
+
 	// Add --wisp flag for ephemeral messages (stored in single DB, filtered from JSONL export)
 	if r.shouldBeWisp(msg) {
 		args = append(args, "--wisp")