Collapse chat delivery into notification actions

[shuxueshuxue]

Summary

• Route chat delivery through RuntimeNotificationAction with chat.message/chat metadata.
• Remove the separate chat runtime delivery gateway surface and obsolete chat delivery adapters.
• Preserve managed-agent typing behavior and external inbox chat wake-token behavior on the notification path.

Verification

• uv run ruff check .
• uv run pyright backend/chat/bootstrap.py backend/threads/bootstrap.py backend/threads/chat_adapters/bootstrap.py backend/threads/chat_adapters/chat_inlet.py backend/threads/chat_adapters/external_inbox_handler.py backend/threads/chat_adapters/gateway.py backend/threads/chat_adapters/port.py backend/threads/chat_adapters/runtime_notification_action.py backend/threads/chat_adapters/thread_handler.py backend/web/core/lifespan.py protocols/agent_runtime.py
• uv run pytest tests/Unit/backend tests/Unit/integration_contracts -q

Notes

• Full repo pyright still reports pre-existing broad typing debt outside this slice, so the PR gate here is targeted pyright plus wider backend/integration unit coverage.
• chat_runtime_notifications remains intentionally; unread chat state is still the durable daemon notification projection.