feat(memory): support multi-vendor embedding fallback

Add embedding_provider config knob with native support for
openai / dashscope / doubao / zhipu / linkai, plus an in-chat
/memory status and /memory rebuild-index workflow for switching
vendors safely.

agent/memory/rebuild_index.py

@@ -0,0 +1,14 @@
+"""
+Backward-compatible shim for the legacy entry point:
+    python -m agent.memory.rebuild_index
+
+The implementation now lives in agent.memory.embedding.rebuild.
+Prefer using `/memory rebuild-index` in chat going forward.
+"""
+
+from agent.memory.embedding.rebuild import main
+
+if __name__ == "__main__":
+    import sys
+
+    sys.exit(main())