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/default.nix

@@ -4,6 +4,7 @@
 {
   pkgs,
   inputs,
+  agents ? null,
 }: {
   # Default shell for working on this repository
   default = pkgs.mkShell {
@@ -32,5 +33,5 @@
   # Import all individual shell environments
   python = import ./python.nix {inherit pkgs inputs;};
   devops = import ./devops.nix {inherit pkgs inputs;};
-  opencode = import ./opencode.nix {inherit pkgs inputs;};
+  coding = import ./coding.nix {inherit pkgs inputs agents;};
 }