test: add basic lib function tests for agents and coding-rules

tests/lib/coding-rules-test.nix

@@ -0,0 +1,53 @@
+let
+  lib = import <nixpkgs/lib>;
+  codingRulesLib = (import ../../lib {inherit lib;}).coding-rules;
+
+  # Test 1: instructions are generated correctly with custom rulesDir
+  testInstructions = let
+    rules = codingRulesLib.mkCodingRules {
+      agents = "/tmp/fake-agents";
+      languages = ["python"];
+      concerns = ["naming"];
+      rulesDir = ".coding-rules";
+    };
+  in
+    assert rules.instructions == [
+      ".coding-rules/concerns/naming.md"
+      ".coding-rules/languages/python.md"
+    ];
+    {result = "pass";};
+
+  # Test 2: default rulesDir is .opencode-rules
+  testDefaultRulesDir = let
+    rules = codingRulesLib.mkCodingRules {
+      agents = "/tmp/fake-agents";
+    };
+    hasCorrectPrefix = builtins.all (s: builtins.substring 0 15 s == ".opencode-rules") rules.instructions;
+  in
+    assert hasCorrectPrefix == true;
+    {result = "pass";};
+
+  # Test 3: backward-compat alias exists
+  testBackwardCompat =
+    assert codingRulesLib.mkOpencodeRules == codingRulesLib.mkCodingRules;
+    {result = "pass";};
+
+  # Test 4: shellHook contains both the symlink command and the config generation
+  testShellHook = let
+    rules = codingRulesLib.mkCodingRules {
+      agents = "/tmp/fake-agents";
+    };
+    hook = rules.shellHook;
+    hasSymlink = builtins.match ".*ln -sfn.*" hook != null;
+    hasConfigGen = builtins.match ".*coding-rules.json.*" hook != null;
+  in
+    assert hasSymlink;
+    assert hasConfigGen;
+    {result = "pass";};
+
+in {
+  instructions-correct = testInstructions;
+  default-rules-dir = testDefaultRulesDir;
+  backward-compat = testBackwardCompat;
+  shell-hook = testShellHook;
+}