feat: add EnvironmentContext summary and update tests

[leslieo2]

Summary

• add an EnvironmentContext helper, gather OS/shell info at runtime, and surface it via ${KIMI_ENV_CONTEXT} in the default system prompt
• slim the context down to the required host facts and update fixtures/tests (default agent, task subagents, env unit test) to normalize the new prompt output
• keep CI green by rerunning ruff, pyright, and targeted pytest suites

Testing

• make check
• pytest tests/test_environment.py tests/test_default_agent.py::test_default_agent tests/test_task_subagents.py::test_task_subagents -vv

[leslieo2]

@codex review