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/embedding/state.py

@@ -0,0 +1,47 @@
+"""
+Embedding-related index utilities.
+
+We don't keep a sidecar state file — the SQLite index is the source of truth
+and config.json is the source of intent. The two functions below are the
+only things needing on-disk awareness:
+
+  detect_index_dim         : read the dim of stored vectors (display-only)
+  cleanup_legacy_state_file: remove old embedding_state.json from earlier
+                             versions; safe no-op when absent.
+"""
+
+from __future__ import annotations
+import json
+import os
+from pathlib import Path
+from typing import Optional, Union
+
+PathLike = Union[str, os.PathLike]
+
+
+def detect_index_dim(storage) -> Optional[int]:
+    """Return the dim of the first stored embedding, or None if the index
+    has no embeddings. Used by /memory status."""
+    try:
+        row = storage.conn.execute(
+            "SELECT embedding FROM chunks WHERE embedding IS NOT NULL LIMIT 1"
+        ).fetchone()
+    except Exception:
+        return None
+    if not row or not row["embedding"]:
+        return None
+    try:
+        emb = json.loads(row["embedding"])
+        return len(emb) if isinstance(emb, list) else None
+    except (json.JSONDecodeError, TypeError):
+        return None
+
+
+def cleanup_legacy_state_file(db_path: PathLike) -> None:
+    """Remove old embedding_state.json files from earlier versions.
+    Safe to call repeatedly; no-op if the file is absent."""
+    legacy = Path(db_path).parent / "embedding_state.json"
+    try:
+        legacy.unlink(missing_ok=True)
+    except Exception:
+        pass