/plugin add should infer marketplace

[Skn0tt]

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: <plugin-name>@<marketplace-name>
   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.