fix(beads): set --type=agent when creating agent beads

CreateAgentBead was creating beads with only --labels=gt:agent but
bd create defaults to --type=task. The bd slot set command requires
type=agent to set slots, causing warnings during gt install and
gt rig add.

Fixes #315

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

internal/beads/beads_agent.go

@@ -136,6 +136,7 @@ func (b *Beads) CreateAgentBead(id, title string, fields *AgentFields) (*Issue,
 		"--id=" + id,
 		"--title=" + title,
 		"--description=" + description,
+		"--type=agent",
 		"--labels=gt:agent",
 	}