support subagent run command parameter

### Describe the feature or problem you'd like to solve

expose the subagent's capabilities directly to the command line

### Proposed solution

_No response_

### Example prompts or workflows

copilot -p "review current changes" --subagent code-review

copilot -p "search Router class" --subagent explore

### Additional context

_No response_

Start by locating the CLI option parser and the code path that launches subagents. Use the two example invocations as acceptance cases: each command should accept --subagent and select the requested code-review or explore capability. Confirm the behavior from the command line and document any validation or error handling needed.

索引モデルが issue の本文から書いたものです。