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

Start by locating the Copilot CLI environment context section and the code that supplies its existing context fields. Trace how runtime environment details are obtained on Linux, macOS, and Windows, then verify that the hostname appears in the context panel while existing fields remain unchanged.

索引モデルが issue の本文から書いたものです。