chore: update flake, agents lib, and clean up tracked dotfiles

- Remove .pi* and .td-root files from git index (now in .gitignore)
- Update flake.lock and flake.nix
- Add shells/coding.nix, remove shells/opencode.nix
- Update lib/agents.nix, lib/coding-rules.nix
- Update modules/home-manager/coding/agents/pi.nix
- Update tests for agents and coding-rules
- Update .gitignore

tests/lib/coding-rules-test.nix

@@ -37,8 +37,74 @@ let
   in
     assert hasSymlink;
     assert hasConfigGen; {result = "pass";};
+
+  # Test 4: forPi=false does not include AGENTS.md logic in shellHook
+  testForPiDisabled = let
+    rules = codingRulesLib.mkCodingRules {
+      agents = "/tmp/fake-agents";
+      forPi = false;
+    };
+    hook = rules.shellHook;
+    hasPiBlock = builtins.match ".*CODING-RULES:START.*" hook != null;
+  in
+    assert hasPiBlock == false; {result = "pass";};
+
+  # Test 5: forPi=true adds CODING-RULES markers to shellHook (when agents path has rules)
+  # Note: This test uses the real AGENTS repo at /home/sascha.koenig/p/AI/AGENTS
+  # It is only run when the path exists.
+  testForPiEnabled = let
+    agentsPath = /home/sascha.koenig/p/AI/AGENTS;
+    rules = codingRulesLib.mkCodingRules {
+      agents = agentsPath;
+      forPi = true;
+      concerns = ["coding-style"];
+      languages = [];
+      frameworks = [];
+    };
+    hook = rules.shellHook;
+    hasPiBlock = builtins.match ".*CODING-RULES:START.*" hook != null;
+    hasCodingStyle = builtins.match ".*Coding Style.*" hook != null;
+  in
+    assert hasPiBlock == true;
+    assert hasCodingStyle == true; {result = "pass";};
+
+  # Test 6: concatRulesMd produces concatenated markdown (with real agents path)
+  testConcatRulesMd = let
+    agentsPath = /home/sascha.koenig/p/AI/AGENTS;
+    md = codingRulesLib.concatRulesMd {
+      agents = agentsPath;
+      concerns = ["coding-style"];
+      languages = [];
+      frameworks = [];
+    };
+    hasHeader = builtins.match ".*Coding Style.*" md != null;
+    hasCritical = builtins.match ".*Critical Rules.*" md != null;
+  in
+    assert hasHeader == true;
+    assert hasCritical == true; {result = "pass";};
+
+  # Test 7: mkRulesMdSection wraps content with markers
+  testRulesMdSection = let
+    agentsPath = /home/sascha.koenig/p/AI/AGENTS;
+    section = codingRulesLib.mkRulesMdSection {
+      agents = agentsPath;
+      concerns = ["coding-style"];
+      languages = [];
+      frameworks = [];
+    };
+    hasStartMarker = builtins.match ".*CODING-RULES:START.*" section != null;
+    hasEndMarker = builtins.match ".*CODING-RULES:END.*" section != null;
+    hasHeader = builtins.match ".*# Coding Rules.*" section != null;
+  in
+    assert hasStartMarker == true;
+    assert hasEndMarker == true;
+    assert hasHeader == true; {result = "pass";};
 in {
   instructions-correct = testInstructions;
   default-rules-dir = testDefaultRulesDir;
   shell-hook = testShellHook;
+  forpi-disabled = testForPiDisabled;
+  forpi-enabled = testForPiEnabled;
+  concat-rules-md = testConcatRulesMd;
+  rules-md-section = testRulesMdSection;
 }