test: align rig beads init fallback coverage

Update the rig init test to assert config.yaml fallback when bd init fails.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

internal/rig/manager.go

@@ -418,7 +418,7 @@ func (m *Manager) initBeads(rigPath, prefix string) error {
 	// Run bd init if available, with --no-agents to skip AGENTS.md creation
 	cmd := exec.Command("bd", "init", "--prefix", prefix, "--no-agents")
 	cmd.Dir = rigPath
-	output, err := cmd.CombinedOutput()
+	_, err := cmd.CombinedOutput()
 	if err != nil {
 		// bd might not be installed or failed, create minimal structure
 		// Note: beads currently expects YAML format for config