Remove the Entire DB credential-store integration

git-sync carried a bespoke Entire credential path: auth.Resolve fell back to an active-user lookup in ~/.config/entire/hosts.json plus a file/keyring token store and OAuth refresh-token handling (client_id=entire-cli). Nothing in the product produces that layout anymore — the mirror-pipeline worker (the only library consumer that talks to entiredb) supplies credentials directly at the transport layer (GitHub installation tokens + per-request entire-core repo-scoped bearers), and client-side entire:// auth is owned by the separate git-remote-entire helper using the newer contexts.json model. So the lookup only ever read a store no current producer writes.

Drop entiredb.go and tokenstore.go and the LookupEntireDBCredential fallback; auth.Resolve now resolves explicit token/bearer credentials only and otherwise returns nil so the git credential helper is consulted on a 401, exactly as for any other remote. This also drops the github.com/zalando/go-keyring dependency and, with the file token store gone, removes the syscall.Flock usage that broke the Windows build (so the package now cross-compiles for windows cleanly).

Supersedes the entiredb token-refresh fix (#90) and the tokenstore Windows flock fix (#92), both of which were polishing this now-deleted code.