refactor: move thread runtime prologue

[shuxueshuxue]

Summary

• move run-source tracking and the user_message/run_start/notice prologue block into backend/thread_runtime/run/prologue.py
• retarget _run_agent_to_buffer to the new prologue owner without changing terminal followthrough/input behavior
• add owner smoke plus direct prologue coverage for owner, steer, and system-notice paths

Verification

• uv run python -m pytest tests/Unit/backend/web/services/test_thread_runtime_owner.py tests/Unit/backend/thread_runtime/run/test_prologue.py tests/Integration/test_query_loop_backend_contracts.py -k "test_run_agent_to_buffer_emits_notice_for_system_agent_notifications or test_run_agent_to_buffer_surfaces_notice_then_assistant_followthrough or test_run_agent_to_buffer_turns_silent_terminal_reentry_into_visible_followthrough" -q
• uv run ruff check backend/thread_runtime/run/prologue.py backend/web/services/streaming_service.py tests/Unit/backend/web/services/test_thread_runtime_owner.py tests/Unit/backend/thread_runtime/run/test_prologue.py
• uv run ruff format --check backend/thread_runtime/run/prologue.py backend/web/services/streaming_service.py tests/Unit/backend/web/services/test_thread_runtime_owner.py tests/Unit/backend/thread_runtime/run/test_prologue.py
• git diff --check