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/witness/handlers.go

@@ -12,6 +12,7 @@ import (
 	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/git"
 	"github.com/steveyegge/gastown/internal/mail"
+	"github.com/steveyegge/gastown/internal/rig"
 	"github.com/steveyegge/gastown/internal/tmux"
 	"github.com/steveyegge/gastown/internal/workspace"
 )
@@ -746,12 +747,12 @@ func AutoNukeIfClean(workDir, rigName, polecatName string) *NukePolecatResult {
 	return result
 }
 
-// verifyCommitOnMain checks if the polecat's current commit is on main.
+// verifyCommitOnMain checks if the polecat's current commit is on the default branch.
 // This prevents nuking a polecat whose work wasn't actually merged.
 //
 // Returns:
-//   - true, nil: commit is verified on main
-//   - false, nil: commit is NOT on main (don't nuke!)
+//   - true, nil: commit is verified on default branch
+//   - false, nil: commit is NOT on default branch (don't nuke!)
 //   - false, error: couldn't verify (treat as unsafe)
 func verifyCommitOnMain(workDir, rigName, polecatName string) (bool, error) {
 	// Find town root from workDir
@@ -760,6 +761,12 @@ func verifyCommitOnMain(workDir, rigName, polecatName string) (bool, error) {
 		return false, fmt.Errorf("finding town root: %v", err)
 	}
 
+	// Get configured default branch for this rig
+	defaultBranch := "main" // fallback
+	if rigCfg, err := rig.LoadRigConfig(filepath.Join(townRoot, rigName)); err == nil && rigCfg.DefaultBranch != "" {
+		defaultBranch = rigCfg.DefaultBranch
+	}
+
 	// Construct polecat path: <townRoot>/<rigName>/polecats/<polecatName>
 	polecatPath := filepath.Join(townRoot, rigName, "polecats", polecatName)
 
@@ -772,16 +779,16 @@ func verifyCommitOnMain(workDir, rigName, polecatName string) (bool, error) {
 		return false, fmt.Errorf("getting polecat HEAD: %w", err)
 	}
 
-	// Verify it's an ancestor of main (i.e., it's been merged)
-	// We use the polecat's git context to check main
-	isOnMain, err := g.IsAncestor(commitSHA, "origin/main")
+	// Verify it's an ancestor of default branch (i.e., it's been merged)
+	// We use the polecat's git context to check
+	isOnDefaultBranch, err := g.IsAncestor(commitSHA, "origin/"+defaultBranch)
 	if err != nil {
 		// Try without origin/ prefix in case remote isn't set up
-		isOnMain, err = g.IsAncestor(commitSHA, "main")
+		isOnDefaultBranch, err = g.IsAncestor(commitSHA, defaultBranch)
 		if err != nil {
-			return false, fmt.Errorf("checking if commit is on main: %w", err)
+			return false, fmt.Errorf("checking if commit is on %s: %w", defaultBranch, err)
 		}
 	}
 
-	return isOnMain, nil
+	return isOnDefaultBranch, nil
 }