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/refinery/engineer.go

@@ -87,12 +87,16 @@ type Engineer struct {
 
 // NewEngineer creates a new Engineer for the given rig.
 func NewEngineer(r *rig.Rig) *Engineer {
+	cfg := DefaultMergeQueueConfig()
+	// Override target branch with rig's configured default branch
+	cfg.TargetBranch = r.DefaultBranch()
+
 	return &Engineer{
 		rig:         r,
 		beads:       beads.New(r.Path),
 		mrQueue:     mrqueue.New(r.Path),
 		git:         git.NewGit(r.Path),
-		config:      DefaultMergeQueueConfig(),
+		config:      cfg,
 		workDir:     r.Path,
 		output:      os.Stdout,
 		eventLogger: mrqueue.NewEventLoggerFromRig(r.Path),