fix(subagents): preserve user id in child runtime context

[yu-xin-c]

Summary

• carry the authenticated user_id from the lead-agent runtime metadata into task subagent execution
• pass user_id through SubagentExecutor into the child agent runtime context so runtime-scoped tools do not fall back to default
• add focused regression coverage for context/metadata extraction and child runtime propagation

Verification

• uv run ruff format packages/harness/deerflow/agents/lead_agent/agent.py packages/harness/deerflow/subagents/executor.py packages/harness/deerflow/tools/builtins/task_tool.py tests/test_task_tool_core_logic.py tests/test_subagent_executor.py
• uv run ruff check packages/harness/deerflow/agents/lead_agent/agent.py packages/harness/deerflow/subagents/executor.py packages/harness/deerflow/tools/builtins/task_tool.py tests/test_task_tool_core_logic.py tests/test_subagent_executor.py
• uv run pytest tests/test_task_tool_core_logic.py tests/test_subagent_executor.py -q — 78 passed

Part of #3102