[haofeng0705]

Summary

When a checksummed release archive contains no usable skills for the selected mode and no local checkout is available, both installers currently return from skills installation and still print Installation complete!.

• Throw a terminating PowerShell error for this branch and for a missing local mono skills directory, preserving the existing finally cleanup.
• Fail through the shell installer's existing err helper for the equivalent release-archive branch.
• Add regression tests using local archives and HTTP stubs, covering mono selection with a multi-only archive, multi selection with no skills, and the PowerShell local-directory failure.
• Add a user-visible release fragment.

The download-failure branch already fails; this change addresses the separate missing-skill-content branch described in #779. PowerShell uses throw rather than the Write-Err helper's exit, to avoid closing an interactive irm | iex session.

Closes #779.

Risk tier

• High-risk under the repository's installer classification.

The implementation change is limited to three failure branches. Automation that previously accepted these false-success results will now receive failures. A full install may already have installed the CLI binary before skills installation fails.

Verification

• Release fragment added: .changes/779-installer-skills-fail-closed.md.

• Static comparison against main@87d23e5cf1b181e33aa302b0502d1af2922cea79: both original installer blobs retain the affected branches.

• Regression tests added and formatted with gofmt; tests have not been run locally.

• Targeted test command:

  DWS_PACKAGE_VERSION=0.0.0-test go test ./test/scripts -run '^TestInstall(Script|PowerShell)MissingSkillsFailsWithoutSuccessBanner$' -count=1

• Native Windows PowerShell 5.1 execution and the existing successful-install tests remain required verification. The PowerShell regression test prefers Windows PowerShell on Windows and skips when no supported PowerShell executable is installed.

• Full suite, packaging verification, builds and policy checks have not been run.

• No CLI command paths, flags, generated artifacts or workflows change.

Notes

Proxy routing, README expansion and broader interactive exit behavior are outside this patch.

[typefield]

CR 结论：改动正确，建议合并（等 CI 全绿）

重点核对

1. 失败时能否中断成功提示 —— 能，两端都成立

• shell：err() 是 printf + exit 1（install.sh:52），在 install_skills 缺失分支直接终止进程，main 里的 🎉 Installation complete! 不会执行；trap EXIT 仍会清理 tmpdir。
• PowerShell：顶层 main 无 try/catch，throw 作为脚本终止性错误直接跳过 banner。Install-Skills 内新增的 throw 位于 try/finally 中，finally 的 $tmpDir 清理保留完好；Install-SkillsLocal 的 throw 无论被 main 直接调用还是经 Install-Skills 的 try/finally，都不会被吞。

2. PowerShell 用 throw 而非 Write-Err —— 理由成立

Write-Err 里是 exit 1，irm | iex 场景会杀掉用户的交互会话；throw 只终止脚本块。同函数内下载失败分支仍用 Write-Err（irm|iex 下会关会话），属于已知不一致，PR notes 已声明范围外，可接受，建议后续统一。

3. 测试覆盖 —— 有效且真实（已代跑）

PR 自述"测试未在本地运行"，我实际执行了目标测试命令：

PR 分支:  5/5 PASS（shell mono/multi + ps1 mono/multi/local-missing；本机有 pwsh，PS 测试真实执行）
main 分支: 如预期 FAIL —— 复现了 "Skipping skills installation" 后仍打印 "🎉 Installation complete!" 的假成功

反向验证通过，说明是真回归测试。fixture 设计到位：SHA256 校验通过后才进入内容检查（断言输出含 "SHA256 checksum verified"），排除的是"内容缺失"而非网络/校验失败；同时断言失败后不创建 ~/.agents/skills/dws。mono 用 multi-only 包、multi 用无 skills 包，两个方向都覆盖到了。

建议（非阻塞）

1. shell 本地目录缺失分支与 ps1 不对称：ps1 的 Install-SkillsLocal 缺目录已改为带 "Skills were NOT installed." 的 throw，但 shell 的 install_skills_local（install.sh 约 959 行）仍是 say "Skipping skills installation." + return 1。set -e 下确实也会 fail-closed、不会打印成功 banner，但用户看到的最后信息是 "Skipping" 而退出码是失败，信息有误导。建议顺手对齐成 err "...Skills were NOT installed."。
2. 小脆性提示：shell 测试 envWithSkillMode 先 append 默认值（DWS_SKILLS_ONLY=0），靠 extras 后追加覆盖，依赖 sh 后者覆盖前者的语义——现有测试已这么用且通过，仅提示。
3. ps1 throw 会带 script position 堆栈噪声，纯美观问题。

其他

• .changes/779-installer-skills-fail-closed.md 符合目录既有约定（category: Fixed）。
• 覆盖缺口提醒：未测"release 包无可用 skills 但本地 source 可用"的 fallback 成功路径，但已有 source-mode 测试覆盖本地安装，可接受。
• Windows PowerShell 5.1 原生验证依赖 CI Windows job（目前 Test (Windows) 已 SUCCESS）。

[typefield]

CI 已全绿。结合上面的 CR（失败分支确实中断成功提示、throw/finally 清理正确、回归测试本地代跑通过且在 main 上反向验证可复现假成功），Approve。剩余 3 条均为非阻塞建议，可后续跟进。