Add sync.branch configuration support (bd-b7d2)

- Created internal/syncbranch package with validation and env var support
- Added --branch flag to bd init command
- Enhanced bd config get/set to validate sync.branch
- Added BEADS_SYNC_BRANCH environment variable support
- Comprehensive tests for branch name validation
- Supports precedence: env var > database config > empty (current branch)

internal/syncbranch/syncbranch.go

@@ -0,0 +1,98 @@
+package syncbranch
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"regexp"
+
+	"github.com/steveyegge/beads/internal/storage"
+)
+
+const (
+	// ConfigKey is the database config key for sync branch
+	ConfigKey = "sync.branch"
+	
+	// EnvVar is the environment variable for sync branch
+	EnvVar = "BEADS_SYNC_BRANCH"
+)
+
+// branchNamePattern validates git branch names
+// Based on git-check-ref-format rules
+var branchNamePattern = regexp.MustCompile(`^[a-zA-Z0-9][a-zA-Z0-9._/-]*[a-zA-Z0-9]$`)
+
+// ValidateBranchName checks if a branch name is valid according to git rules
+func ValidateBranchName(name string) error {
+	if name == "" {
+		return nil // Empty is valid (means use current branch)
+	}
+	
+	// Basic length check
+	if len(name) > 255 {
+		return fmt.Errorf("branch name too long (max 255 characters)")
+	}
+	
+	// Check pattern
+	if !branchNamePattern.MatchString(name) {
+		return fmt.Errorf("invalid branch name: must start and end with alphanumeric, can contain .-_/ in middle")
+	}
+	
+	// Disallow certain patterns
+	if name == "HEAD" || name == "." || name == ".." {
+		return fmt.Errorf("invalid branch name: %s is reserved", name)
+	}
+	
+	// No consecutive dots
+	if regexp.MustCompile(`\.\.`).MatchString(name) {
+		return fmt.Errorf("invalid branch name: cannot contain '..'")
+	}
+	
+	// No leading/trailing slashes
+	if name[0] == '/' || name[len(name)-1] == '/' {
+		return fmt.Errorf("invalid branch name: cannot start or end with '/'")
+	}
+	
+	return nil
+}
+
+// Get retrieves the sync branch configuration with the following precedence:
+// 1. BEADS_SYNC_BRANCH environment variable
+// 2. sync.branch from database config
+// 3. Empty string (meaning use current branch)
+func Get(ctx context.Context, store storage.Storage) (string, error) {
+	// Check environment variable first
+	if envBranch := os.Getenv(EnvVar); envBranch != "" {
+		if err := ValidateBranchName(envBranch); err != nil {
+			return "", fmt.Errorf("invalid %s: %w", EnvVar, err)
+		}
+		return envBranch, nil
+	}
+	
+	// Check database config
+	dbBranch, err := store.GetConfig(ctx, ConfigKey)
+	if err != nil {
+		return "", fmt.Errorf("failed to get %s from config: %w", ConfigKey, err)
+	}
+	
+	if dbBranch != "" {
+		if err := ValidateBranchName(dbBranch); err != nil {
+			return "", fmt.Errorf("invalid %s in database: %w", ConfigKey, err)
+		}
+	}
+	
+	return dbBranch, nil
+}
+
+// Set stores the sync branch configuration in the database
+func Set(ctx context.Context, store storage.Storage, branch string) error {
+	if err := ValidateBranchName(branch); err != nil {
+		return err
+	}
+	
+	return store.SetConfig(ctx, ConfigKey, branch)
+}
+
+// Unset removes the sync branch configuration from the database
+func Unset(ctx context.Context, store storage.Storage) error {
+	return store.DeleteConfig(ctx, ConfigKey)
+}