feat: scaffold codespace backend

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/process/opencode.go

@@ -0,0 +1,12 @@
+package process
+
+// DefaultOpenCodeCommand is the default command used to launch OpenCode.
+const DefaultOpenCodeCommand = "opencode"
+
+// normalizeCommand returns command if non-empty, otherwise the default.
+func normalizeCommand(command string) string {
+	if command == "" {
+		return DefaultOpenCodeCommand
+	}
+	return command
+}