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

lib/agents.nix

@@ -223,7 +223,10 @@
       canonical,
       modelOverrides ? {},
       primaryAgent ? null,
+      codingRules ? null,
     }: let
+      # Import coding-rules lib for concatRulesMd when codingRules is provided
+      codingRulesLib = (import ./coding-rules.nix {inherit lib;});
       # Find the primary agent (there should be exactly one).
       primaryAgents = lib.filterAttrs (_: a: a.mode == "primary") canonical;
       primaryNames = lib.attrNames primaryAgents;
@@ -306,6 +309,17 @@
           "- **" + dn + "**: " + agent.description;
       in
         lib.mapAttrsToList mkEntry subagents;
+      # ── Coding rules section (optional) ────────────────────────
+      # When codingRules is provided, append selected rules to AGENTS.md.
+      # codingRules attrset: { agents, languages, concerns, frameworks }
+      codingRulesSection =
+        if codingRules != null
+        then let
+          section = codingRulesLib.mkRulesMdSection codingRules;
+        in
+          if section != "" then "\n" + section else ""
+        else "";
+
       agentsMd =
         "# Agent Instructions\n"
         + "\n"
@@ -320,7 +334,8 @@
           if subagents == {}
           then ""
           else "## Available Specialists\n\n" + lib.concatStringsSep "\n" specialistEntries + "\n"
-        );
+        )
+        + codingRulesSection;
 
       agentsMdFile = pkgs.writeText "AGENTS.md" agentsMd;
       systemMdFile = pkgs.writeText "SYSTEM.md" primary.systemPrompt;
@@ -346,6 +361,7 @@
       agentsInput,
       tool,
       modelOverrides ? {},
+      codingRules ? null,
     }: let
       canonical = agentsInput.lib.loadAgents;
     in
@@ -362,7 +378,7 @@
       else if tool == "pi"
       then
         agentsLib.renderForPi {
-          inherit pkgs canonical modelOverrides;
+          inherit pkgs canonical modelOverrides codingRules;
         }
       else throw "lib.agents.renderForTool: unknown tool '${tool}'. Must be opencode, claude-code, or pi.";
 
@@ -386,9 +402,10 @@
       agentsInput,
       tool,
       modelOverrides ? {},
+      codingRules ? null,
     }: let
       rendered = agentsLib.renderForTool {
-        inherit pkgs agentsInput tool modelOverrides;
+        inherit pkgs agentsInput tool modelOverrides codingRules;
       };
     in
       if tool == "opencode"