fix(release): default unsigned MSIX in CI for Store submission

[11cookies11]

[中文]

• 变更内容：

  • release workflow 默认将 MSIX_SKIP_SIGN 设为 true（仅当未配置 secret 时）

  • build_msix.ps1 在使用自签证书时，验签失败改为告警，不再导致 CI 失败

  • 更新 where progress 记录本次任务状态

• 影响范围：

  • .github/workflows/release.yml

  • scripts/build_msix.ps1

  • .where-agent-progress.md

• 兼容性/行为变化：

  • 默认产物改为未签名 MSIX（适用于上传 Microsoft Store 由平台签名）

  • 提供正式证书时仍可按原流程签名

• 依赖/环境：

  • 无新增依赖

[English]

• Changes:

  • Set MSIX_SKIP_SIGN to true by default in release workflow when secret is empty

  • Downgrade self-signed SignTool verification failures to warnings in build_msix.ps1

  • Update where progress file for this task

• Impact:

  • .github/workflows/release.yml

  • scripts/build_msix.ps1

  • .where-agent-progress.md

• Compatibility / Behavior Changes:

  • CI now produces unsigned MSIX by default for Store submission

  • Signed flow remains available when a real certificate is provided

• Dependencies / Environment:

  • No new dependencies