feat: add DeepSeekHarness studio agent

[zakahan]

Summary

• add DeepSeekHarness as a Studio built-in sandbox agent entry while reusing the Codex CodeEnv tool IDs
• add session metadata separation and same-origin surface proxy rewriting for DeepSeekHarness WebUI/API/assets
• bind MODEL_AGENT_API_KEY, MODEL_AGENT_NAME, and MODEL_AGENT_BASE_URL onto CodeEnv tools for DSH

Tests

• uv run pre-commit run --all-files
• uv run pytest tests/cli/test_frontend_sandbox.py tests/cli/test_frontend_agent_proxy.py tests/cli/test_frontend_skill_creator.py tests/cli/test_frontend_sandbox_options.py -q
• uv run pytest tests/cli/test_studio_deploy_target.py::test_sandbox_snapshot_tool_id_must_differ_from_standard_tool_id tests/cli/test_studio_deploy_target.py::test_studio_deploy_byteplus_auto_provisions_four_sandbox_tools tests/cli/test_studio_update.py::test_studio_update_only_overrides_explicit_sandbox_tool_id tests/cli/test_studio_update.py::test_byteplus_studio_update_repairs_missing_sandbox_tools -q
• npm test
• npm run build