feat(rig): Add --branch flag for custom default branch

Add --branch flag to `gt rig add` to specify a custom default branch
instead of auto-detecting from remote. This supports repositories that
use non-standard default branches like `develop` or `release`.

Changes:
- Add --branch flag to `gt rig add` command
- Store default_branch in rig config.json
- Propagate default branch to refinery, witness, daemon, and all commands
- Rename ensureMainBranch to ensureDefaultBranch for clarity
- Add Rig.DefaultBranch() method for consistent access
- Update crew/manager.go and swarm/manager.go to use rig config

Based on PR #49 by @kustrun - rebased and extended with additional fixes.

Co-authored-by: kustrun <kustrun@users.noreply.github.com>

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

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

internal/cmd/start.go

@@ -750,8 +750,8 @@ func runStartCrew(cmd *cobra.Command, args []string) error {
 		fmt.Printf("Crew workspace %s/%s exists\n", rigName, name)
 	}
 
-	// Ensure crew workspace is on main branch
-	ensureMainBranch(worker.ClonePath, fmt.Sprintf("Crew workspace %s/%s", rigName, name))
+	// Ensure crew workspace is on default branch
+	ensureDefaultBranch(worker.ClonePath, fmt.Sprintf("Crew workspace %s/%s", rigName, name), r.Path)
 
 	// Resolve account for Claude config
 	accountsPath := constants.MayorAccountsPath(townRoot)
@@ -933,8 +933,8 @@ func startCrewMember(rigName, crewName, townRoot string) error {
 		return fmt.Errorf("getting crew worker: %w", err)
 	}
 
-	// Ensure crew workspace is on main branch
-	ensureMainBranch(worker.ClonePath, fmt.Sprintf("Crew workspace %s/%s", rigName, crewName))
+	// Ensure crew workspace is on default branch
+	ensureDefaultBranch(worker.ClonePath, fmt.Sprintf("Crew workspace %s/%s", rigName, crewName), r.Path)
 
 	// Create tmux session
 	t := tmux.NewTmux()