fix(nix): stop automatic rebuilds and nightly GC from tripping GitHub's rate limit

autoUpgrade was silently failing daily on both hosts (legacy channel path,
no --flake set) without reaching the network — disabled outright since
rebuilds should only happen manually. Separately, nix-gc.timer's nightly
sweep was deleting eval-time-only flake input sources (not part of the
runtime closure), forcing the next evaluation to cold-refetch hermes-agent's
entire dependency tree from GitHub in one burst, which alone was enough to
hit the unauthenticated codeload rate limit. keep-outputs/keep-derivations
stop GC from evicting those sources.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TJS8qsYPXSUm5mdWGdaHjF

machines/jupiter/configuration.nix

@@ -72,6 +72,9 @@
 
   system = {
     stateVersion = "23.05";
-    autoUpgrade.enable = true;
+    # Was silently failing daily (legacy channel path, no --flake set) without
+    # ever reaching the network. Disabled rather than fixed: rebuilds should
+    # only happen when triggered manually.
+    autoUpgrade.enable = false;
   };
 }