fix: update troubleshooting guide for outdated Antigravity User-Agent and process management

docs/TROUBLESHOOTING.md

@@ -30,6 +30,39 @@ rm ~/.config/opencode/antigravity-accounts.json
 opencode auth login
 ```
 
+### "This version of Antigravity is no longer supported"
+This almost always means an outdated Antigravity `User-Agent` is still being used.
+
+1) Stop any running OpenCode processes (stale processes can overwrite your accounts file):
+
+**macOS/Linux:**
+```bash
+pkill -f opencode || true
+```
+
+**Windows (PowerShell):**
+```powershell
+Stop-Process -Name "opencode" -Force -ErrorAction SilentlyContinue
+```
+
+2) Clear the plugin caches and re-login:
+
+**macOS/Linux:**
+```bash
+rm -f ~/.config/opencode/antigravity-accounts.json
+rm -rf ~/.cache/opencode/node_modules/opencode-antigravity-auth
+rm -rf ~/.bun/install/cache/opencode-antigravity-auth*
+opencode auth login
+```
+
+**Windows (PowerShell):**
+```powershell
+Remove-Item "$env:APPDATA\opencode\antigravity-accounts.json" -Force -ErrorAction SilentlyContinue
+Remove-Item "$env:LOCALAPPDATA\opencode\Cache\node_modules\opencode-antigravity-auth" -Recurse -Force -ErrorAction SilentlyContinue
+Remove-Item "$env:USERPROFILE\.bun\install\cache\opencode-antigravity-auth*" -Recurse -Force -ErrorAction SilentlyContinue
+opencode auth login
+```
+
 ### "Model not found"
 Add this to your `google` provider config:
 ```json
@@ -45,7 +78,7 @@ The correct key is `plugin` (singular):
 
 ```json
 {
-  "plugin": ["opencode-antigravity-auth@beta"]
+  "plugin": ["opencode-antigravity-auth@latest"]
 }
 ```
 
@@ -398,4 +431,4 @@ npx tsx script/test-regression.ts --dry-run     # List tests
 
 ## Still stuck?
 
-Open an issue on [GitHub](https://github.com/NoeFabris/opencode-antigravity-auth/issues).
+Open an issue on [GitHub](https://github.com/NoeFabris/opencode-antigravity-auth/issues).