feat(opencode): Windows PowerShell installer and Desktop key file

CLI and Desktop share %USERPROFILE%\.config\opencode. Persist the API
key in .arka-api-key so GUI apps do not need a login-shell export.

README.md

@@ -10,12 +10,18 @@
 |-------|-----|
 | [`opencode/`](opencode/) | Провайдер OpenCode **Arka-AI**: установщик, плагин, skill, каталог чат-моделей |
 
-Однострочник OpenCode:
+Однострочник OpenCode (macOS/Linux/WSL):
 
 ```bash
 curl -fsSL https://git.arka-ai.ru/arka/public/raw/branch/main/opencode/install-arka-provider.sh | bash
 ```
 
+Windows (PowerShell, CLI и Desktop):
+
+```powershell
+irm https://git.arka-ai.ru/arka/public/raw/branch/main/opencode/install-arka-provider.ps1 | iex
+```
+
 ---
 
 Public Arka AI downloads, docs, and scripts. Anonymous raw URLs, no token.
@@ -28,8 +34,14 @@ One top-level folder per product. No private source.
 |--------|------|
 | [`opencode/`](opencode/) | OpenCode **Arka-AI** provider: installer, plugin, skill, chat catalog |
 
-OpenCode one-liner:
+OpenCode one-liner (macOS/Linux/WSL):
 
 ```bash
 curl -fsSL https://git.arka-ai.ru/arka/public/raw/branch/main/opencode/install-arka-provider.sh | bash
 ```
+
+Windows (PowerShell, CLI and Desktop):
+
+```powershell
+irm https://git.arka-ai.ru/arka/public/raw/branch/main/opencode/install-arka-provider.ps1 | iex
+```