Clean up TODOs and verify custom-template item edits

[jscarle]

Summary

• remove stale TODO entries that referred to already-merged PR and issue triage work
• add regression coverage to verify EditItem(...) preserves a custom-template item category_id in the edit payload
• clarify in the README and quick-start guide that the hydrate-edit-save flow also applies to custom-template items

Testing

• dotnet build OnePassword.NET.sln -c Release
• dotnet test OnePassword.NET.sln -c Release