auth: delete the legacy keyring store and pre-contexts migration (COR-393)

contexts.json + the shared tokenstore are now the only credential store. Login records the context fatally instead of dual-writing a legacy entire-cli/<authBaseURL> keyring slot; the context-preferring ContextStore wrapper, CurrentContextToken, LookupCurrentToken, and MigrateLegacyLoginContext (with its callers in the git remote helper and the repo-token path) are gone. Pre-contexts logins now surface the existing "run entire login" hints instead of being bridged.

The authfilestore build tag existed only to keep the legacy store off developer keychains in tests; the surviving store honors ENTIRE_TOKEN_STORE=file unconditionally, so the tag, the file backend, and the keyring-timeout shim go too. Integration login tests move to --server + the v2 device-authorization path and sandbox via ENTIRE_CONFIG_DIR / ENTIRE_TOKEN_STORE.