Implement JWT token auth with transparent auto-renewal.

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>

proto/sgard/v1/sgard.proto

@@ -82,8 +82,32 @@ message PruneResponse {
   int32 blobs_removed = 1;
 }
 
+// Auth messages.
+
+message AuthenticateRequest {
+  bytes  nonce      = 1; // 32-byte nonce (server-provided or client-generated)
+  int64  timestamp  = 2; // Unix seconds
+  bytes  signature  = 3; // SSH signature over (nonce || timestamp)
+  string public_key = 4; // SSH public key in authorized_keys format
+}
+
+message AuthenticateResponse {
+  string token = 1; // JWT valid for 30 days
+}
+
+// ReauthChallenge is embedded in Unauthenticated error details when a
+// token is expired but was previously valid. The client signs this
+// challenge to obtain a new token without generating its own nonce.
+message ReauthChallenge {
+  bytes nonce     = 1; // server-generated 32-byte nonce
+  int64 timestamp = 2; // server's current Unix timestamp
+}
+
 // GardenSync is the sgard remote sync service.
 service GardenSync {
+  // Authenticate exchanges an SSH-signed challenge for a JWT token.
+  rpc Authenticate(AuthenticateRequest) returns (AuthenticateResponse);
+
   // Push flow: send manifest, then stream missing blobs.
   rpc PushManifest(PushManifestRequest) returns (PushManifestResponse);
   rpc PushBlobs(stream PushBlobsRequest) returns (PushBlobsResponse);