fix: installation via volta broken due to symbolic links pointing to tmp file

The PR uses a relative path for symbolic link

See https://github.com/sst/opencode/ssues/197

Fix verified locally by running local npm registry:

```
npx verdaccio@latest --listen 4873
```

And publishing opencode to that:

```
OPENCODE_VERSION="1.0.0-local" OPENCODE_DRY="false" bun run script/publish.ts
```

The `script/publish.ts` was updated to publish to local registry by appending
`--registry http://localhost:4873` to the `bun install` commands.

Volta was then used to install from the local registry:

```
NPM_CONFIG_REGISTRY=http://localhost:4873 npm_config_registry=http://localhost:4873 volta install [email protected]
```

Author: mrloop

packages/opencode/script/postinstall.mjs

@@ -79,7 +79,7 @@ function main() {
     }
 
     // Create symlink to the actual binary
-    fs.symlinkSync(binaryPath, binScript)
+    fs.symlinkSync(path.relative(path.dirname(binScript), binaryPath), binScript)
     console.log(`opencode binary symlinked: ${binScript} -> ${binaryPath}`)
   } catch (error) {
     console.error("Failed to create opencode binary symlink:", error.message)