Fix EverCore demo memory payload

[hylarucoder]

Summary

• update demo/extract_memory.py to send top-level content for V1 MessageItem payloads
• keep the fix scoped to the demo without changing server DTO compatibility

Verification

• PYTHONPATH=src:. uv run python - <<'PY' ... validated that the new demo payload passes PersonalAddRequest and the old text: {content} payload fails at messages.0.content
• git diff --check

Fixes #223