/plugin add should infer marketplace

Running this works in Claude Code, but not in Copilot CLI:

```
/plugin marketplace add microsoft/playwright-cli
/plugin install playwright-cli
```

Copilot fails with

```
✗ Invalid plugin specification: "playwright-cli"

Format: @
Example: document-skills@anthropic-agent-skills
```

If the plugin name is unique across all installed marketplaces, I think it should be installable without fully qualifying the marketplace name.