fix(core): narrow backtest subprocess environment

[warren618]

Summary

• narrow generated-backtest subprocess environment inheritance to OS/Python basics, proxy/certificate settings, allowed run-root config, and read-only market-data settings
• avoid forwarding LLM provider keys, API auth tokens, shell-tool switches, broker trading secrets, and live/advisory toggles into generated strategy code
• document the generated backtest trust boundary in README and SECURITY

Closes #332

Validation

• PYTHONPATH=agent pytest agent/tests/test_runner_env.py agent/tests/test_run_card.py agent/tests/test_path_safety.py agent/tests/test_file_tool_sandbox_security.py -q
• python -m ruff check agent/src/core/runner.py agent/tests/test_runner_env.py
• python -m compileall -q agent/src/core/runner.py agent/tests/test_runner_env.py
• git diff --check