fix(installer): avoid PromptScript failure for DSH

[zzAllenn]

Summary

• install the AgentKey Skill through the Skills CLI universal target for DSH-only installs
• avoid passing either dsh or the unsupported global PromptScript target while preserving explicit mixed-agent targets
• update Bash, PowerShell, English/Chinese setup guidance, retry instructions, and release-asset source docs
• add Bash mixed-target coverage and a Windows PowerShell invocation regression

Root cause

skills@1.5.23 -g -y adds PromptScript to its universal target set even though PromptScript has no global install directory. The canonical AgentKey Skill still installs, but the command prints Failed to install 1. Explicitly targeting universal writes the same ~/.agents/skills/agentkey copy without selecting PromptScript.

Validation

• bats tests/ — 42/42
• bash -n scripts/install.sh
• bash -n scripts/uninstall.sh
• bash -n scripts/build-release-assets.sh
• scripts/build-release-assets.sh with an isolated output directory
• workflow YAML parse
• git diff --check
• Windows behavior regression added to the windows-latest Scripts tests job

Deployment follow-up

Merging/releasing this PR does not automatically update agentkey.app/install.sh. After release, publish the current Bash/PowerShell install and uninstall scripts to the website origin and purge the CDN; the live endpoints were serving stale pre-DSH content during verification.