Plugin package with a subpath export fails to install (pkg/subpath parsed as GitHub repo)

Description

A plugin configured with an npm subpath export fails to install. opencode-pty ships a V2 entrypoint as opencode-pty/v2 (via exports: { "./v2": ... }), but the spec is parsed as a GitHub repo instead of an npm subpath.

Reproducing the parse directly:

$ bun add opencode-pty/v2
error: GET https://api.github.com/repos/opencode-pty/v2/tarball/ - 404
error: opencode-pty/v2 failed to resolve

Via the server (opencode plugin add opencode-pty/v2 or "plugins": [{"package": "opencode-pty/v2"}]) the log shows NpmInstallFailedError with An unknown git error occurred.

Cause: Npm.isInstallablePackage (packages/util/src/npm.ts) resolves package specs with npm-package-arg, which parses name/subpath as the GitHub shorthand github:name/subpath. Expected behaviour would be to install the registry root and import the full requested specifier (same gap as the closed #16586; related #47517).

Plugins

opencode-pty@0.4.0 (configured spec opencode-pty/v2)

OpenCode version

2.0.8

Steps to reproduce

1. opencode plugin add opencode-pty/v2 (or add "plugins": [{"package": "opencode-pty/v2"}] to opencode.json)
2. Server logs NpmInstallFailedError / An unknown git error occurred; the plugin never loads.
3. Isolate the parse: bun add opencode-pty/v2 → GET https://api.github.com/repos/opencode-pty/v2/tarball/ - 404

Operating System

Fedora Linux (x64)

Terminal

not relevant to the bug (server-side install path)

Start in packages/util/src/npm.ts at Npm.isInstallablePackage and inspect how npm-package-arg parses the plugin specifier. Reproduce the failure with bun add opencode-pty/v2 and trace the server-side plugin install path. Done means opencode-pty/v2 is treated as an npm package with a subpath rather than a GitHub repository, and the plugin installation succeeds.