fix: use persistent platform-appropriate dirs instead of /tmp for vector store defaults (#4279)

Replace hardcoded /tmp/{provider} default paths with XDG-compliant persistent
directories (~/.local/share/mem0/{provider}), fixing failures on Windows,
macOS LaunchAgents, Linux systemd, and Docker. Adds MEM0_DATA_DIR env var
override, legacy /tmp data migration warning, and updates Qdrant/ChromaDB/FAISS
configs and docs to use the new defaults.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/open-source/overview.mdx

@@ -77,7 +77,7 @@ Mem0 Open Source delivers the same adaptive memory engine as the platform, but p
   Mem0 OSS works out of the box with sensible defaults:
   - LLM: OpenAI `gpt-4.1-nano-2025-04-14` (via `OPENAI_API_KEY`)
   - Embeddings: OpenAI `text-embedding-3-small`
-  - Vector store: Local Qdrant instance storing data at `/tmp/qdrant`
+  - Vector store: Local Qdrant instance storing data at `~/.local/share/mem0/qdrant`
   - History store: SQLite database at `~/.mem0/history.db`
   - Reranker: Disabled until you configure a provider
 

docs/open-source/python-quickstart.mdx

@@ -81,7 +81,7 @@ print(results)
 By default `Memory()` wires up:
 - OpenAI `gpt-4.1-nano-2025-04-14` for fact extraction and updates
 - OpenAI `text-embedding-3-small` embeddings (1536 dimensions)
-- Qdrant vector store with on-disk data at `/tmp/qdrant`
+- Qdrant vector store with on-disk data at `~/.local/share/mem0/qdrant`
 - SQLite history at `~/.mem0/history.db`
 - No reranker (add one in the config when you need it)
 </Note>