refactor(runtime): restructure runs module with new execution architecture

Major refactoring of deerflow/runtime/:
- runs/callbacks/ - new callback system (builder, events, title, tokens)
- runs/internal/ - execution internals (executor, supervisor, stream_logic, registry)
- runs/internal/execution/ - execution artifacts and events handling
- runs/facade.py - high-level run facade
- runs/observer.py - run observation protocol
- runs/types.py - type definitions
- runs/store/ - simplified store interfaces (create, delete, query, event)

Refactor stream_bridge/:
- Replace old providers with contract.py and exceptions.py
- Remove async_provider.py, base.py, memory.py

Add documentation:
- README.md and README_zh.md for runtime module

Remove deprecated:
- manager.py moved to internal/
- worker.py, schemas.py
- user_context.py

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/packages/harness/deerflow/runtime/serialization.py

@@ -4,8 +4,8 @@ Provides a single source of truth for converting LangChain message
 objects, Pydantic models, and LangGraph state dicts into plain
 JSON-serialisable Python structures.
 
-Consumers: ``deerflow.runtime.runs.worker`` (SSE publishing) and
-``app.gateway.routers.threads`` (REST responses).
+Consumers: runs execution internals (SSE publishing) and
+gateway thread state/history responses.
 """
 
 from __future__ import annotations