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.

opencode/skills/arka-provider/SKILL.md

@@ -24,7 +24,8 @@ It is wired up locally with a plugin + a static `provider` block in
 - Static block (`opencode.json` → `provider["Arka-AI"]`) holds
   `name`, `api`, `env`, and a small fallback `models` map that is used when the
   catalog is unreachable.
-- Inference still uses `ARKA_API_KEY` against `https://api.arka-ai.ru/v1`.
+- Inference uses `ARKA_API_KEY` (env) or `~/.config/opencode/.arka-api-key`
+  (so OpenCode Desktop / Windows GUI works without a login shell).
 
 ## Refresh the model list