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/opencode-rules.nix

@@ -0,0 +1,110 @@
+# Opencode rules management utilities
+#
+# This module provides functions to configure Opencode agent rules across
+# multiple projects. Rules are defined in the AGENTS repository and can be
+# selectively included based on language, framework, and concerns.
+#
+# Usage in your configuration:
+#
+#   # In your flake or configuration:
+#   let
+#     m3taLib = inputs.m3ta-nixpkgs.lib.${system};
+#
+#     rules = m3taLib.opencode-rules.mkOpencodeRules {
+#       agents = inputs.agents;
+#       languages = [ "python" "typescript" ];
+#       concerns = [ "coding-style" "naming" "documentation" ];
+#       frameworks = [ "react" "fastapi" ];
+#     };
+#   in {
+#     # Use in your devShell:
+#     devShells.default = pkgs.mkShell {
+#       shellHook = rules.shellHook;
+#       inherit (rules) instructions;
+#     };
+#   }
+#
+# The shellHook creates:
+#   - A `.opencode-rules/` symlink pointing to the AGENTS repository rules directory
+#   - An `opencode.json` file with a $schema reference and instructions list
+#
+# The instructions list contains paths relative to the project root, all prefixed
+# with `.opencode-rules/`, making them portable across different project locations.
+{ lib }: {
+  # Create Opencode rules configuration from AGENTS repository
+  #
+  # Args:
+  #   agents: Path to the AGENTS repository (non-flake input)
+  #   languages: Optional list of language-specific rules to include
+  #              (e.g., [ "python" "typescript" "rust" ])
+  #   concerns: Optional list of concern rules to include
+  #             Default: [ "coding-style" "naming" "documentation" "testing" "git-workflow" "project-structure" ]
+  #   frameworks: Optional list of framework-specific rules to include
+  #               (e.g., [ "react" "fastapi" "django" ])
+  #   extraInstructions: Optional list of additional instruction paths
+  #                      (for custom rules outside standard locations)
+  #
+  # Returns:
+  #   An attribute set containing:
+  #     - shellHook: Bash code to create symlink and opencode.json
+  #     - instructions: List of rule file paths (relative to project root)
+  #
+  # Example:
+  #   mkOpencodeRules {
+  #     agents = inputs.agents;
+  #     languages = [ "python" ];
+  #     frameworks = [ "fastapi" ];
+  #   }
+  #   # Returns:
+  #   # {
+  #   #   shellHook = "...";
+  #   #   instructions = [
+  #   #     ".opencode-rules/concerns/coding-style.md"
+  #   #     ".opencode-rules/concerns/naming.md"
+  #   #     ".opencode-rules/concerns/documentation.md"
+  #   #     ".opencode-rules/concerns/testing.md"
+  #   #     ".opencode-rules/concerns/git-workflow.md"
+  #   #     ".opencode-rules/concerns/project-structure.md"
+  #   #     ".opencode-rules/languages/python.md"
+  #   #     ".opencode-rules/frameworks/fastapi.md"
+  #   #   ];
+  #   # }
+  mkOpencodeRules = { agents, languages ? [ ], concerns ? [
+    "coding-style"
+    "naming"
+    "documentation"
+    "testing"
+    "git-workflow"
+    "project-structure"
+  ], frameworks ? [ ], extraInstructions ? [ ] }:
+    let
+      rulesDir = ".opencode-rules";
+
+      # Build instructions list by mapping concerns, languages, frameworks to their file paths
+      # All paths are relative to project root via the rulesDir symlink
+      instructions = (map (c: "${rulesDir}/concerns/${c}.md") concerns)
+        ++ (map (l: "${rulesDir}/languages/${l}.md") languages)
+        ++ (map (f: "${rulesDir}/frameworks/${f}.md") frameworks)
+        ++ extraInstructions;
+
+      # Generate JSON configuration for Opencode
+      opencodeConfig = {
+        "$schema" = "https://opencode.ai/config.json";
+        inherit instructions;
+      };
+    in {
+      inherit instructions;
+
+      # Shell hook to set up rules in the project
+      # Creates a symlink to the AGENTS rules directory and generates opencode.json
+      shellHook = ''
+        # Create/update symlink to AGENTS rules directory
+        ln -sfn ${agents}/rules ${rulesDir}
+
+        # Generate opencode.json configuration file
+        cat > opencode.json <<'OPENCODE_EOF'
+        ${builtins.toJSON opencodeConfig}
+        OPENCODE_EOF
+      '';
+    };
+}