Add archcore login/logout with OS keychain token storage

[ivklgn]

Problem

Sync has no first-class authentication flow and no secure token storage.

Expected result

archcore login / archcore logout commands that store the auth token in the OS keychain and let sync reuse it, never writing it in plaintext settings.

Impact

Gives sync a secure, standard authentication UX across platforms.

Scope

• Add login / logout commands.
• Store/retrieve the token via the OS keychain (cross-platform).
• Tests around the token lifecycle (mock the keychain).

Files: cmd/login.go, cmd/logout.go (new) + tests

Acceptance: users authenticate once and sync reuses the stored token.

Status: blocked — the sync feature is currently gated off at the command layer.