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/templates/templates.go

@@ -29,6 +29,7 @@ type RoleData struct {
 	TownRoot       string   // e.g., "/Users/steve/ai"
 	TownName       string   // e.g., "ai" - the town identifier for session names
 	WorkDir        string   // current working directory
+	DefaultBranch  string   // default branch for merges (e.g., "main", "develop")
 	Polecat        string   // polecat name (for polecat role)
 	Polecats       []string // list of polecats (for witness role)
 	BeadsDir       string   // BEADS_DIR path