Plugin loader always attempts npm install, failing for local/non-published packages

[henengelh]

Problem

When a plugin listed in opencode.json does not exist on npm, the plugin loader
fails with a 404 error and silently swallows the plugin — providers login then
returns "Unknown provider" with no actionable message.

The loader unconditionally runs:
opencode add --force --exact --ignore-scripts --cwd ~/.cache/opencode @latest

even when the package is already present in node_modules or when a local path is intended.

Reproduction

1. Create a local plugin at ~/.config/opencode/node_modules/my-plugin/
2. Add "plugin": ["my-plugin"] to opencode.json
3. Run opencode providers login --provider <id-registered-by-plugin>
4. Result: "Unknown provider" (npm 404 kills loading silently)

Workaround

Use the absolute path to the plugin directory instead of the package name:

"plugin": ["/Users/<you>/.config/opencode/node_modules/my-plugin"]                                                                                                                                               

This skips the npm install step and loads directly from disk.

Expected behaviour

• If the plugin value is an absolute/relative path, skip npm and load directly.
• If npm install fails, fall back to an existing local copy if present.
• Surface the 404/install failure to the user with a clear error message
  rather than "Unknown provider".

Environment

opencode 1.3.13, macOS