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/rig/types.go

@@ -79,3 +79,13 @@ func (r *Rig) BeadsPath() string {
 	}
 	return r.Path
 }
+
+// DefaultBranch returns the configured default branch for this rig.
+// Falls back to "main" if not configured or if config cannot be loaded.
+func (r *Rig) DefaultBranch() string {
+	cfg, err := LoadRigConfig(r.Path)
+	if err != nil || cfg.DefaultBranch == "" {
+		return "main"
+	}
+	return cfg.DefaultBranch
+}