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/daemon/lifecycle.go

@@ -11,6 +11,7 @@ import (
 	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/config"
 	"github.com/steveyegge/gastown/internal/constants"
+	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/tmux"
 )
@@ -467,6 +468,20 @@ func (d *Daemon) applySessionTheme(sessionName string, parsed *ParsedIdentity) {
 // syncWorkspace syncs a git workspace before starting a new session.
 // This ensures agents with persistent clones (like refinery) start with current code.
 func (d *Daemon) syncWorkspace(workDir string) {
+	// Determine default branch from rig config
+	// workDir is like <townRoot>/<rigName>/<role>/rig or <townRoot>/<rigName>/crew/<name>
+	defaultBranch := "main" // fallback
+	rel, err := filepath.Rel(d.config.TownRoot, workDir)
+	if err == nil {
+		parts := strings.Split(rel, string(filepath.Separator))
+		if len(parts) > 0 {
+			rigPath := filepath.Join(d.config.TownRoot, parts[0])
+			if rigCfg, err := rig.LoadRigConfig(rigPath); err == nil && rigCfg.DefaultBranch != "" {
+				defaultBranch = rigCfg.DefaultBranch
+			}
+		}
+	}
+
 	// Fetch latest from origin
 	fetchCmd := exec.Command("git", "fetch", "origin")
 	fetchCmd.Dir = workDir
@@ -475,7 +490,7 @@ func (d *Daemon) syncWorkspace(workDir string) {
 	}
 
 	// Pull with rebase to incorporate changes
-	pullCmd := exec.Command("git", "pull", "--rebase", "origin", "main")
+	pullCmd := exec.Command("git", "pull", "--rebase", "origin", defaultBranch)
 	pullCmd.Dir = workDir
 	if err := pullCmd.Run(); err != nil {
 		d.logger.Printf("Warning: git pull failed in %s: %v", workDir, err)