Investigate removing Darc storing PATs on local disk in base64

As a follow-up item from the 1CS review, investigate whether Darc can avoid storing PATs (Personal Access Tokens) on local disk encoded in base64.

## Context
Part of the 1CS follow-up work tracked in the parent epic. See https://github.com/dotnet/arcade-services/issues/6448 for related 1CS workitems.

Storing PATs on local disk, even base64-encoded, is not a secure storage mechanism since base64 is trivially reversible and offers no real protection. This should be investigated as a potential security improvement.

## Tasks
- [ ] Identify where Darc currently stores PATs on local disk and how they're encoded
- [ ] Investigate alternative secure storage options (e.g., OS-level credential managers/keychains)
- [ ] Assess feasibility and impact of changing the storage mechanism
- [ ] Propose and document a recommended approach