fix: move blocking storage I/O off event loop

[Upper9527]

Summary

• run session object-store writes through asyncio.to_thread
• run periodic skill pulls through asyncio.to_thread
• keep reload and logging behavior on the event-loop thread
• add regression coverage that both synchronous storage calls cross the worker-thread boundary

Closes #52.

Verification

• pytest -q tests/test_async_storage_io.py tests/test_session_upload_trigger.py — 11 passed
• uvx ruff@0.12.2 check .
• uvx ruff@0.12.2 format --check .
• full suite: 1 pre-existing dashboard failure, 132 passed, 1 skipped