Entra backend resolves OAuth tokens via MSAL client_credentials
(OBO flow wired in future sprint) and passes ACs through for
Bascule. Kerberos stubbed pending hybrid environment config.
Stub backend for dev/testing without real IdP.
Signed-off-by: Tyler King <tking@guildhouse.dev>
Zero-credential-storage architecture. The broker holds ACs
(authorization). Pluggable CredentialBackend implementations
hold secrets. Transports acquire short-lived, scoped credentials
at invocation time and discard after use.
Credential types: BasculeCredential, KerberosCredential,
OAuthCredential, SSHCertCredential.
Signed-off-by: Tyler King <tking@guildhouse.dev>