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/agents-test.nix

@@ -20,7 +20,67 @@ let
     result = agentsLib.loadCanonical {agentsInput = fakeInput;};
   in
     assert result == {test = {description = "test";};}; {result = "pass";};
+
+  # Test 3: renderForPi accepts codingRules parameter without error (null case)
+  # Verifies that passing codingRules = null produces the same result as omitting it.
+  # Uses a minimal fake canonical set instead of a real agents repo.
+  testPiNullCodingRules = let
+    pkgs = import <nixpkgs> {};
+    canonical = {
+      chiron = {
+        mode = "primary";
+        description = "Test primary agent";
+        display_name = "Chiron";
+        systemPrompt = "You are a test agent.";
+        permissions = {};
+      };
+    };
+    result = agentsLib.renderForPi {
+      inherit pkgs canonical;
+      codingRules = null;
+    };
+    agentsMd = builtins.readFile "${result}/AGENTS.md";
+    hasMarkers = builtins.match ".*CODING-RULES:START.*" agentsMd != null;
+  in
+    assert hasMarkers == false; {result = "pass";};
+
+  # Test 4: renderForPi with codingRules includes rules in AGENTS.md
+  # Uses the real AGENTS repo to read rule files (requires --impure or local path)
+  testPiWithCodingRules = let
+    agentsPath = /home/sascha.koenig/p/AI/AGENTS;
+    pkgs = import <nixpkgs> {};
+    canonical = {
+      chiron = {
+        mode = "primary";
+        description = "Test primary agent";
+        display_name = "Chiron";
+        systemPrompt = "You are a test agent.";
+        permissions = {};
+      };
+    };
+    result = agentsLib.renderForPi {
+      inherit pkgs canonical;
+      codingRules = {
+        agents = agentsPath;
+        concerns = ["coding-style"];
+        languages = [];
+        frameworks = [];
+      };
+    };
+    agentsMd = builtins.readFile "${result}/AGENTS.md";
+    hasStartMarker = builtins.match ".*CODING-RULES:START.*" agentsMd != null;
+    hasEndMarker = builtins.match ".*CODING-RULES:END.*" agentsMd != null;
+    hasCodingStyle = builtins.match ".*Coding Style.*" agentsMd != null;
+    # Also verify agent descriptions are still present
+    hasAgentInstructions = builtins.match ".*Agent Instructions.*" agentsMd != null;
+  in
+    assert hasStartMarker == true;
+    assert hasEndMarker == true;
+    assert hasCodingStyle == true;
+    assert hasAgentInstructions == true; {result = "pass";};
 in {
   unknown-tool-throws = testUnknownTool;
   load-canonical = testLoadCanonical;
+  pi-null-coding-rules = testPiNullCodingRules;
+  pi-with-coding-rules = testPiWithCodingRules;
 }