[Bug] install.ps1 prints "🎉 Installation complete!" even when skills installation silently fails #779

Describe the Bug

When running install.ps1 with DWS_SKILLS_ONLY=1 to install skills separately, the skills download can fail (Gitee gateway 502/503 or unreachable GitHub), but the script only prints two easily-missed warning lines and still ends with 🎉 Installation complete! and exit code 0. As a result, ~\.agents\skills\dws stays empty while the user believes the installation succeeded.

Steps to Reproduce

1. Install the binary only:

   $env:DWS_NO_SKILLS = "1"
   irm https://raw.githubusercontent.com/DingTalk-Real-AI/dingtalk-workspace-cli/main/scripts/install.ps1 | iex

2. Later, try to install skills only:

   $env:DWS_SKILLS_ONLY = "1"
   irm https://raw.githubusercontent.com/DingTalk-Real-AI/dingtalk-workspace-cli/main/scripts/install.ps1 | iex

3. If the dws-skills.zip download fails (likely for mainland-China users: the GitHub probe in Resolve-Source connects to github.com directly, and the Gitee API gateway intermittently returns 502/503), the script prints:

   ⚠️  Release asset download failed. Trying local source...
   ⚠️  No local source found either. Skipping skills installation.
   

   ...and then still prints 🎉 Installation complete!.

Expected Behavior

• Skills are installed to ~\.agents\skills\dws, or
• the script fails loudly: prints the warnings via Write-Err, skips the success banner, and exits with a non-zero code.

Actual Behavior

~\.agents\skills\dws remains empty. The script ends with:

🎉 Installation complete!

Next steps:
  dws --help           # explore commands

The only failure indication is two ⚠️ warning lines in the middle of the output, which are easy to miss.

Environment

• OS: Windows 11
• Architecture: amd64
• CLI Version: dws v1.0.54 (windows/amd64)
• Install method: irm .../scripts/install.ps1 | iex via gh-proxy mirror (GitHub is unreachable directly)

Additional Context

Root cause: in Install-Skills, when the release asset download fails the script falls back to a local source checkout; when that is also missing (always the case for irm | iex), it prints a warning and returns — but the main flow unconditionally prints the success banner and exits 0.

Verified separately that the Gitee asset itself is fine: https://gitee.com/dingtalk-real-ai/dingtalk-workspace-cli/releases/download/v1.0.54/dws-skills.zip (2.3 MB, contains mono/SKILL.md).

Suggestions:

1. Track the skills-install result and exit non-zero (or at least skip the 🎉 banner) on failure.
2. Honor HTTPS_PROXY / add a DWS_PROXY option for the GitHub probe and asset downloads.
3. Consider a dws skill install subcommand so users can (re)install skills without re-running install.ps1.
4. Document DWS_SKILLS_ONLY / DWS_NO_SKILLS / DWS_INSTALL_DIR in the README.