feat: add crew directory structure and manager

Add crew/ directory support to rig structure for user-managed
persistent workspaces. Crew workers are separate from polecats
(AI-managed) and can have optional custom BEADS_DIR configuration.

- Add internal/crew package with Worker type and Manager
- Update rig types to include Crew slice and CrewCount in summary
- Update rig manager to scan for crew workers
- Add crew/ to AgentDirs for rig initialization

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

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

internal/rig/manager.go

@@ -94,6 +94,16 @@ func (m *Manager) loadRig(name string, entry config.RigEntry) (*Rig, error) {
 		}
 	}
 
+	// Scan for crew workers
+	crewDir := filepath.Join(rigPath, "crew")
+	if entries, err := os.ReadDir(crewDir); err == nil {
+		for _, e := range entries {
+			if e.IsDir() {
+				rig.Crew = append(rig.Crew, e.Name())
+			}
+		}
+	}
+
 	// Check for witness
 	witnessPath := filepath.Join(rigPath, "witness", "rig")
 	if _, err := os.Stat(witnessPath); err == nil {