Route agent inputs through runtime gateway

[shuxueshuxue]

Summary

• add Agent Runtime Gateway envelopes for chat delivery and direct thread input
• route Chat delivery and Thread message entrypoints through the gateway
• delete legacy message_routing service and core.agents.communication delivery package
• move chat delivery wiring into backend/web/services/chat_delivery_adapter.py

Verification

• uv run pytest tests/Unit/backend/web/services/test_chat_delivery_adapter.py tests/Unit/backend/web/services/test_agent_runtime_gateway_chat_delivery.py tests/Unit/backend/web/services/test_agent_runtime_gateway_thread_input.py tests/Integration/test_threads_router.py tests/Integration/test_query_loop_backend_contracts.py tests/Integration/test_messaging_social_handle_contract.py tests/Integration/test_messaging_router.py -q
• uv run pytest tests/Unit/backend/web/services tests/Unit/core/test_agent_pool.py tests/Unit/core/test_agent_service.py tests/Integration/test_threads_router.py tests/Integration/test_query_loop_backend_contracts.py tests/Integration/test_messaging_social_handle_contract.py tests/Integration/test_messaging_router.py -q
• uv run ruff check .
• uv run pyright backend/web/services/agent_runtime_gateway.py backend/web/services/chat_delivery_adapter.py backend/web/core/lifespan.py backend/web/routers/threads.py tests/Unit/backend/web/services/test_chat_delivery_adapter.py tests/Unit/backend/web/services/test_agent_runtime_gateway_chat_delivery.py tests/Unit/backend/web/services/test_agent_runtime_gateway_thread_input.py
• git diff --check

Note: full-repo pyright still fails on existing unrelated type debt outside this slice.