fix(refinery): use rig's default_branch instead of hardcoded 'main'

- Add DefaultBranch field to RoleData struct
- Update refinery.md.tmpl to use {{ .DefaultBranch }} template variable
- Populate DefaultBranch from rig config in prime.go and rig/manager.go
- Default to 'main' if not configured
- Add test verifying DefaultBranch rendering in refinery template

Fixes issue where refinery agents merged to master instead of the
configured default branch (e.g., 'develop' or 'develop-cstar').

internal/cmd/install.go

@@ -271,6 +271,7 @@ func createMayorCLAUDEmd(hqRoot, townRoot string) error {
 		TownRoot:      townRoot,
 		TownName:      townName,
 		WorkDir:       hqRoot,
+		DefaultBranch: "main", // Mayor doesn't merge, but field required
 		MayorSession:  session.MayorSessionName(),
 		DeaconSession: session.DeaconSessionName(),
 	}