fix: refresh Notion cache when opening a new session

[mikemikimike]

Summary

• clear persisted Notion page-cache entries when a plugin session opens
• keep same-session pagination caching while preventing stale cross-session results
• add a regression test for cache removal

Fixes #69

Testing

• git diff --check passes
• go test ./... is blocked in this environment because downloading github.com/julien040/anyquery@v0.1.6 from proxy.golang.org failed with a network connection error