test,ci: 建立 phpunit 基建 + 移植 WC 測試 + CI workflow

- composer.json / phpunit.xml / tests/bootstrap.php（比照 HP AddOn，
  直接 require 核心 plugin 的 unit bootstrap）
- tests/unit/WooCommerceIntegrationTest.php（自 A 移植，10 tests）
- .gitea/workflows/test.yml：lint + unit（unit job 會一併 checkout 核心 plugin，
  因為測試 bootstrap 依賴它）

10 tests / 33 assertions GREEN

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TbG1keQQ7XBa7qMQY16KCY

phpunit.xml

@@ -0,0 +1,21 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="tests/bootstrap.php"
+         colors="true"
+         failOnWarning="true"
+         beStrictAboutOutputDuringTests="true"
+         cacheDirectory=".phpunit.cache">
+    <testsuites>
+        <testsuite name="woocommerce-addon">
+            <directory>tests/unit</directory>
+        </testsuite>
+    </testsuites>
+    <source>
+        <include>
+            <directory>includes</directory>
+            <directory>admin</directory>
+            <directory>cli</directory>
+        </include>
+    </source>
+</phpunit>