auth: fix OIDCVerifier injection wiring + OIDCDevStub root attachment
Surfaced by the first end-to-end smoke of `auth.oidc.enabled=true`:
- `OIDCVerifier.Cfg` was typed `config.OIDCConfig`, but only `AuthConfig`
is registered as a `config-section:""` (config.go:31-37). Steward's
type-keyed injection threw `failed to find dependency` at `mgr.Inject`,
panicking the daemon. Latent since 80b1c09 — never reached because no
prior verified path enabled OIDC. Retyped to `config.AuthConfig`,
Init now reads `Cfg.OIDC.{Issuer,Audience,UsernameClaim}`. Sibling-
consistent with `Authenticator` and `OIDCDevStub`.
- `OIDCDevStub` had no dependents and no `steward.Root()` modifier, so
steward logged `ERR empty dependents asset without root option` and
skipped it in lifecycle bookkeeping (Destroy never called on shutdown).
Added `steward.Root()` to its registration; shutdown log now shows
`destroying component component=auth.OIDCDevStub`.