ci: add upstream sync workflows and optimize Docker CI builds

- Add auto-upstream-sync workflow with dedup protection for conflict issues
- Add upstream-sync build + smoke test workflow
- Add smoke-test script and test files
- Add upstream sync issue template and documentation
- Fix: use --allow-unrelated-histories for merge command
- Fix: check for existing open merge-conflict issues before creating duplicates

.github/test-files/test.txt

@@ -0,0 +1,6 @@
+Hello, this is a smoke test file for ConvertX.
+這是 ConvertX 的冒煙測試檔案。
+日本語テスト：こんにちは
+한국어 테스트: 안녕하세요
+
+This file should be less than 1KB for quick testing.