Replace per-call SSH signing with a two-layer auth system:
Server: AuthInterceptor verifies JWT tokens (HMAC-SHA256 signed with
repo-local jwt.key). Authenticate RPC accepts SSH-signed challenges
and issues 30-day JWTs. Expired-but-valid tokens return a
ReauthChallenge in error details (server-provided nonce for fast
re-auth). Authenticate RPC is exempt from token requirement.
Client: TokenCredentials replaces SSHCredentials as the primary
PerRPCCredentials. NewWithAuth creates clients with auto-renewal —
EnsureAuth obtains initial token, retryOnAuth catches Unauthenticated
errors and re-authenticates transparently. Token cached at
$XDG_STATE_HOME/sgard/token (0600).
CLI: dialRemote() helper handles token loading, connection setup,
and initial auth. Push/pull/prune commands simplified to use it.
Proto: Added Authenticate RPC, AuthenticateRequest/Response,
ReauthChallenge messages.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
