copilot plugin install --help misses local path case

`copilot plugin install --help` lists four source formats but omits local paths. According to [docs](https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-finding-installing#install-from-a-local-path) these are supported: the help should also list `./path/to/plugin - Install from a local directory`

My scenario was working on plugin/skills in a local repo and wanting to try loading them.