openspec: drop pass-through, superseded by llm-agents

openspec is now consumed from numtide/llm-agents.nix via the
shared-nixpkgs overlay (pkgs.llm-agents.openspec), which is fresher than
the separately pinned Fission-AI input. Removes the duplicate source of
truth; drops the now-unused flake input.

pkgs/default.nix

@@ -2,7 +2,6 @@
 #
 # Flake inputs used:
 #   inputs.basecamp  → basecamp (pass-through)
-#   inputs.openspec → openspec (pass-through)
 #   inputs.opencode → opencode-desktop (Electron desktop app, pure pass-through)
 #   inputs.agents   → not used directly here (used by lib/)
 {
@@ -52,7 +51,6 @@ in {
   #   __intentionallyOverridingVersion = true;
   # });
   basecamp = inputs.basecamp.packages.${system}.default;
-  openspec = inputs.openspec.packages.${system}.default;
   opencode-desktop = inputs.opencode.packages.${system}.opencode-desktop;
 
   # buzz: Block's "hive mind" desktop app. Consumed from mulatta/buzz.nix