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

modules/home-manager/coding/agents/pi.nix

@@ -44,6 +44,60 @@ in
         '';
       };
 
+      codingRules = mkOption {
+        type = types.nullOr (types.submodule {
+          options = {
+            languages = mkOption {
+              type = types.listOf types.str;
+              default = [];
+              description = ''
+                Language-specific coding rules to include
+                (e.g. [ "python" "typescript" "nix" ]).
+                Rule files are read from the AGENTS repo's rules/languages/ directory.
+              '';
+            };
+
+            concerns = mkOption {
+              type = types.listOf types.str;
+              default = [
+                "coding-style"
+                "naming"
+                "documentation"
+                "testing"
+                "git-workflow"
+                "project-structure"
+              ];
+              description = ''
+                Concern rules to include from the AGENTS repo's rules/concerns/ directory.
+              '';
+            };
+
+            frameworks = mkOption {
+              type = types.listOf types.str;
+              default = [];
+              description = ''
+                Framework-specific coding rules to include
+                (e.g. [ "react" "fastapi" ]).
+                Rule files are read from the AGENTS repo's rules/frameworks/ directory.
+              '';
+            };
+          };
+        });
+        default = null;
+        description = ''
+          Coding rules to inject into ~/.pi/agent/AGENTS.md.
+          Rules are read from the AGENTS repository and appended as markdown sections.
+          Requires agentsInput to be set.
+        '';
+        example = literalExpression ''
+          {
+            languages = [ "python" "typescript" ];
+            concerns = [ "coding-style" "testing" ];
+            frameworks = [ "fastapi" ];
+          }
+        '';
+      };
+
       settings = mkOption {
         type = types.submodule {
           freeformType = types.attrsOf types.anything;
@@ -166,6 +220,12 @@ in
 
       piSettings = filterNulls cfg.settings;
 
+      # Coding rules config for renderForPi (only when both agentsInput and codingRules are set)
+      piCodingRules =
+        if cfg.agentsInput != null && cfg.codingRules != null
+        then cfg.codingRules // { agents = cfg.agentsInput; }
+        else null;
+
       # Rendered agents (only computed when agentsInput is set)
       rendered =
         if cfg.agentsInput != null
@@ -175,6 +235,7 @@ in
             canonical = cfg.agentsInput.lib.loadAgents;
             modelOverrides = cfg.modelOverrides;
             primaryAgent = cfg.primaryAgent;
+            codingRules = piCodingRules;
           }
         else null;