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(),
 	}

internal/cmd/prime.go

@@ -19,6 +19,7 @@ import (
 	"github.com/steveyegge/gastown/internal/constants"
 	"github.com/steveyegge/gastown/internal/events"
 	"github.com/steveyegge/gastown/internal/lock"
+	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/style"
 	"github.com/steveyegge/gastown/internal/templates"
@@ -308,12 +309,22 @@ func outputPrimeContext(ctx RoleContext) error {
 	// Get town name for session names
 	townName, _ := workspace.GetTownName(ctx.TownRoot)
 
+	// Get default branch from rig config (default to "main" if not set)
+	defaultBranch := "main"
+	if ctx.Rig != "" && ctx.TownRoot != "" {
+		rigPath := filepath.Join(ctx.TownRoot, ctx.Rig)
+		if rigCfg, err := rig.LoadRigConfig(rigPath); err == nil && rigCfg.DefaultBranch != "" {
+			defaultBranch = rigCfg.DefaultBranch
+		}
+	}
+
 	data := templates.RoleData{
 		Role:          roleName,
 		RigName:       ctx.Rig,
 		TownRoot:      ctx.TownRoot,
 		TownName:      townName,
 		WorkDir:       ctx.WorkDir,
+		DefaultBranch: defaultBranch,
 		Polecat:       ctx.Polecat,
 		MayorSession:  session.MayorSessionName(),
 		DeaconSession: session.DeaconSessionName(),