fix: expand tilde in global gitignore path from git config

git config --global core.excludesfile may return paths like ~/...
which Go does not expand. This caused setupGlobalGitIgnore to fail
when users had configured their gitignore with a tilde path.

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

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/init.go

@@ -1198,6 +1198,14 @@ func setupGlobalGitIgnore(homeDir string, verbose bool) error {
 	if err == nil && len(output) > 0 {
 		// User has already configured a global gitignore file, use it
 		ignorePath = strings.TrimSpace(string(output))
+
+		// Expand tilde if present (git config may return ~/... which Go doesn't expand)
+		if strings.HasPrefix(ignorePath, "~/") {
+			ignorePath = filepath.Join(homeDir, ignorePath[2:])
+		} else if ignorePath == "~" {
+			ignorePath = homeDir
+		}
+
 		if verbose {
 			fmt.Printf("Using existing configured global gitignore file: %s\n", ignorePath)
 		}