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

.gitignore

@@ -43,5 +43,5 @@ flake.lock.bak
 .sidecar-start.sh
 .sidecar-base
 .td-root
-.pi-lens
 .cache
+.pi*