Show local machine hostname in Copilot CLI interface

## Summary
Please include the local machine hostname in the Copilot CLI interface context panel.

## Motivation
When running multiple shells/machines (local + remote, containers, SSH sessions), it is easy to lose track of where a command will run. Showing hostname directly in the interface would reduce mistakes.

## Requested behavior
Add a field in the environment context section, e.g.:

- `Hostname: my-machine`

## Notes
- Prefer the actual runtime hostname from the executing environment
- Keep existing context fields unchanged
- This should work across Linux/macOS/Windows