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

shells/coding.nix

@@ -0,0 +1,111 @@
+# AI coding agent development environment with coding rules
+# Sets up coding rules for OpenCode and Pi, plus useful companion tools.
+# Usage: nix develop .#coding
+#
+# To enable coding rules, add the agents input to your flake:
+#   agents = {
+#     url = "git+https://code.m3ta.dev/m3tam3re/AGENTS";
+#     flake = false;
+#   };
+{
+  pkgs,
+  lib ? pkgs.lib,
+  inputs ? null,
+  agents ? null,
+}: let
+  # Import the coding-rules library
+  m3taLib = import ../lib {lib = pkgs.lib;};
+
+  # Import custom packages
+  customPackages = import ../pkgs {inherit pkgs inputs;};
+
+  # Create rules configuration only if agents input is provided
+  rulesConfig = lib.optionalAttrs (agents != null) {
+    rules = m3taLib.coding-rules.mkCodingRules {
+      inherit agents;
+
+      # Languages relevant to this repository
+      languages = ["nix" "python" "shell"];
+
+      # Frameworks used in this repo
+      frameworks = ["n8n"];
+
+      # Standard concerns for development
+      concerns = [
+        "coding-style"
+        "naming"
+        "documentation"
+        "testing"
+        "git-workflow"
+        "project-structure"
+      ];
+
+      # Also append rules to AGENTS.md for Pi agent discovery
+      forPi = true;
+    };
+  };
+in
+  pkgs.mkShell {
+    name = "coding";
+
+    # Development tools
+    buildInputs = with pkgs;
+      [
+        # Task management for AI coding sessions
+        customPackages.td
+
+        # Companion tool for CLI agents (diffs, file trees, task management)
+        customPackages.sidecar
+
+        # Code analysis tools
+        customPackages.code2prompt
+
+        # Nix development tools (for this repo)
+        nil
+        alejandra
+        statix
+        deadnix
+      ];
+
+    shellHook = ''
+      echo "🤖 AI Coding Environment"
+      echo ""
+
+      ${
+        if (agents != null)
+        then ''
+          # Set up coding rules for OpenCode + Pi
+          ${rulesConfig.rules.shellHook}
+
+          echo "✅ Coding rules configured (OpenCode + Pi)"
+          echo "   Languages: nix, python, shell"
+          echo "   Frameworks: n8n"
+          echo "   Concerns: coding-style, naming, documentation, testing, git-workflow, project-structure"
+        ''
+        else ''
+          echo "⚠️  Coding rules not configured"
+          echo ""
+          echo "To enable, add the agents input to your flake.nix:"
+          echo ""
+          echo "  agents = {"
+          echo "    url = \"git+https://code.m3ta.dev/m3tam3re/AGENTS\";"
+          echo "    flake = false;"
+          echo "  };"
+        ''
+      }
+
+      echo ""
+      echo "Available tools:"
+      echo "  opencode              - AI coding agent"
+      echo "  td usage --new-session - View current tasks"
+      echo "  sidecar               - Companion tool (diffs, file trees, tasks)"
+      echo "  code2prompt           - Convert code to prompts"
+      echo ""
+      echo "Nix development tools:"
+      echo "  nix flake check    - Check flake validity"
+      echo "  nix fmt .          - Format Nix files"
+      echo "  statix check .     - Lint Nix files"
+      echo "  deadnix .          - Find dead code"
+      echo ""
+    '';
+  }