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

flake.lock

@@ -1,5 +1,21 @@
 {
   "nodes": {
+    "agents": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1776092721,
+        "narHash": "sha256-avV4Snqp0K57I9s8D61+GHlg9DYZFSIvjaS4d4RYpG8=",
+        "ref": "refs/heads/master",
+        "rev": "0ad41acb03eee0e22cba611b2171a3d3ee30cb10",
+        "revCount": 72,
+        "type": "git",
+        "url": "https://code.m3ta.dev/m3tam3re/AGENTS"
+      },
+      "original": {
+        "type": "git",
+        "url": "https://code.m3ta.dev/m3tam3re/AGENTS"
+      }
+    },
     "basecamp": {
       "inputs": {
         "nixpkgs": [
@@ -96,6 +112,7 @@
     },
     "root": {
       "inputs": {
+        "agents": "agents",
         "basecamp": "basecamp",
         "nixpkgs": "nixpkgs",
         "nixpkgs-master": "nixpkgs-master",