Fix: Remove hard-coded 'main' branch default to allow blank branch field

Previously, CheckRepoSeeding() always defaulted to 'main' when the branch
parameter was nil or empty. This prevented users from leaving the branch
field blank to signal that the runner should create a feature branch.

Changed branchName initialization from "main" to empty string, allowing
the system to properly detect when no branch is specified.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: sallyom

components/backend/git/operations.go

@@ -176,7 +176,7 @@ func getSecretKeys(data map[string][]byte) []string {
 // CheckRepoSeeding checks if a repo has been seeded by verifying .claude/commands/ and .specify/ exist
 // Supports both GitHub and GitLab repositories
 func CheckRepoSeeding(ctx context.Context, repoURL string, branch *string, token string) (bool, map[string]interface{}, error) {
-	branchName := "main"
+	var branchName string
 	if branch != nil && strings.TrimSpace(*branch) != "" {
 		branchName = strings.TrimSpace(*branch)
 	}