fix(package): add ./server export for OpenCode-Go plugin compatibility

OpenCode-Go's resolveExportPath checks exports["./server"] before
falling back to the main field. Without this entry, fs.realpathSync
resolves the entry path relative to CWD instead of the package
directory, triggering a containment security check failure:

  ERROR path=oh-my-opencode error=Plugin resolved server entry outside plugin directory

Fixes #2966

package.json

@@ -18,6 +18,7 @@
       "types": "./dist/index.d.ts",
       "import": "./dist/index.js"
     },
+    "./server": "./dist/index.js",
     "./schema.json": "./dist/oh-my-opencode.schema.json"
   },
   "scripts": {