fix: resolve native addon dependencies for Desktop tool

[NicholasDominici]

Summary

• Add transitive @nut-tree-fork dependencies (libnut, libnut-darwin, shared) to packages/opencode/package.json so native .node bindings resolve correctly at runtime
• Improve error handling in desktop.ts to surface the actual underlying error instead of a generic message, making debugging easier

Problem

The Desktop tool (@nut-tree-fork/nut-js) failed to load with the error:

Desktop automation library not available. Please ensure @nut-tree-fork/nut-js is installed.

Even though @nut-tree-fork/nut-js was installed, its transitive native addon dependencies (@nut-tree-fork/libnut, @nut-tree-fork/libnut-darwin, etc.) were not declared as direct dependencies. Bun's package manager didn't properly symlink these into the packages/opencode/node_modules directory, causing the dynamic require() calls inside the library to fail.

Changes

packages/opencode/package.json

Added missing transitive dependencies:

"@nut-tree-fork/libnut": "4.2.6",
"@nut-tree-fork/libnut-darwin": "2.7.5",
"@nut-tree-fork/shared": "4.2.6"

packages/opencode/src/tool/desktop.ts

• Added error caching to avoid repeated failed imports
• Error messages now include the actual underlying error (e.g., MODULE_NOT_FOUND) instead of a generic message
• Added platform/arch logging for easier debugging