feat(lib): add opencode-rules helper for per-project rule injection

- Create lib/opencode-rules.nix with mkOpencodeRules function
- Update lib/default.nix to import opencode-rules module
- Pattern follows ports.nix: {lib}: { mkOpencodeRules = ...; }

Refs: T2, T3 of rules-system plan

lib/default.nix

@@ -3,10 +3,10 @@
 #   let
 #     m3taLib = inputs.m3ta-nixpkgs.lib.${system};
 #   in ...
-{lib}: {
+{ lib }: {
   # Port management utilities
-  ports = import ./ports.nix {inherit lib;};
+  ports = import ./ports.nix { inherit lib; };
 
-  # Add more helper modules here as needed
-  # example = import ./example.nix { inherit lib; };
+  # OpenCode rules injection utilities
+  opencode-rules = import ./opencode-rules.nix { inherit lib; };
 }