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/swarm/manager.go

@@ -88,7 +88,7 @@ func (m *Manager) LoadSwarm(epicID string) (*Swarm, error) {
 		EpicID:       epicID,
 		BaseCommit:   baseCommit,
 		Integration:  fmt.Sprintf("swarm/%s", epicID),
-		TargetBranch: "main",
+		TargetBranch: m.rig.DefaultBranch(),
 		State:        state,
 		Workers:      []string{}, // Discovered from active tasks
 		Tasks:        []SwarmTask{},