fix(opencode-plugin): support Windows config paths and zip extraction

[freelanceagent1]

Summary

This PR fixes two Windows-specific issues in the OpenCode plugin:

1. Runtime config lookup now resolves the OpenCode config directory the same way as the CLI helpers/doctor path logic.
2. ONNX Runtime ZIP extraction no longer depends on unzip being available on Windows.

Changes

• packages/opencode-plugin/src/config.ts
  • honor OPENCODE_CONFIG_DIR
  • use homedir() instead of process.env.HOME || "~"
• packages/opencode-plugin/src/onnx-runtime.ts
  • for Windows ZIP extraction, try PowerShell Expand-Archive first
  • fall back to cmd.exe /c tar -xf ...
  • keep unzip for non-Windows platforms

Why

1. Runtime config mismatch on Windows

The plugin runtime was using process.env.HOME to derive ~/.config/opencode, while the doctor/config-path helpers used proper platform-aware home directory resolution. On Windows this could lead to doctor seeing a valid AFT config while the live runtime ignored it.

2. ONNX extraction failure on Windows

The semantic-search bootstrap path used unzip -q ... for ZIP archives. On a standard Windows host without unzip in PATH, ONNX download succeeded but extraction failed immediately.

Validation

• built packages/opencode-plugin successfully from source
• verified the built runtime contains the Windows extraction path (Expand-Archive + cmd /c tar -xf fallback)
• exercised the local source build through OpenCode with a temporary config
• confirmed ONNX Runtime downloaded and extracted successfully on Windows instead of failing on unzip

[ualtinok]

Thanks for the clean contribution @freelanceagent1! Merged and will ship in the next patch release.