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"

lib/coding-rules.nix

@@ -27,6 +27,7 @@
 # The shellHook creates:
 #   - A `.opencode-rules/` symlink pointing to the AGENTS repository rules directory
 #   - A `coding-rules.json` file with a $schema reference and instructions list
+#   - (Optional) Appends coding rules to `AGENTS.md` for Pi agent discovery
 #
 # The instructions list contains paths relative to the project root, all prefixed
 # with `.opencode-rules/`, making them portable across different project locations.
@@ -43,6 +44,9 @@
   #               (e.g., [ "react" "fastapi" "django" ])
   #   extraInstructions: Optional list of additional instruction paths
   #                      (for custom rules outside standard locations)
+  #   forPi: Whether to also append rules to AGENTS.md for Pi agent (default: true)
+  #          Pi discovers AGENTS.md files by walking parent dirs + cwd and concatenates them.
+  #          When enabled, a delimited block is appended to (or created in) AGENTS.md.
   #
   # Returns:
   #   An attribute set containing:
@@ -83,6 +87,7 @@
     frameworks ? [],
     extraInstructions ? [],
     rulesDir ? ".opencode-rules",
+    forPi ? false,
   }: let
     # Build instructions list by mapping concerns, languages, frameworks to their file paths
     # All paths are relative to project root via the rulesDir symlink
@@ -97,11 +102,46 @@
       "$schema" = "https://opencode.ai/config.json";
       inherit instructions;
     };
+
+    # Pi rules content (concatenated markdown) — only computed when forPi is true
+    piRulesSection =
+      if forPi
+      then mkRulesMdSection {inherit agents languages concerns frameworks;}
+      else "";
+
+    # Bash snippet to append rules to AGENTS.md for Pi discovery.
+    # Uses HTML comment markers for idempotent updates:
+    #   - Removes any existing CODING-RULES block
+    #   - Appends the new block
+    #   - Creates AGENTS.md if it doesn't exist
+    # Note: Uses plain if-then-else instead of lib.optionalString to avoid
+    # forcing the `lib` argument (which may come from import <nixpkgs/lib>)
+    # when forPi is false.
+    piShellHook =
+      if forPi && piRulesSection != ""
+      then ''
+        # Pi agent: append coding rules to AGENTS.md
+        if [ -f AGENTS.md ]; then
+          # Remove existing coding-rules block (if any)
+          sed -i '/<!-- CODING-RULES:START -->/,/<!-- CODING-RULES:END -->/d' AGENTS.md
+          # Append new coding-rules block
+          cat >> AGENTS.md <<'PIRULES_EOF'
+        ${piRulesSection}
+        PIRULES_EOF
+        else
+          # Create AGENTS.md with just the coding rules
+          cat > AGENTS.md <<'PIRULES_EOF'
+        ${piRulesSection}
+        PIRULES_EOF
+        fi
+      ''
+      else "";
   in {
     inherit instructions;
 
     # Shell hook to set up rules in the project
     # Creates a symlink to the AGENTS rules directory and generates coding-rules.json
+    # Optionally appends rules to AGENTS.md for Pi agent discovery
     shellHook = ''
       # Create/update symlink to AGENTS rules directory
       ln -sfn ${agents}/rules ${rulesDir}
@@ -110,8 +150,73 @@
       cat > coding-rules.json <<'RULES_EOF'
       ${builtins.toJSON rulesConfig}
       RULES_EOF
+
+      ${piShellHook}
     '';
   };
+  # Concatenate selected rule files from the AGENTS repository into a single
+  # markdown string. Used by Pi (append to AGENTS.md) and could be used by
+  # other tools that don't support an instructions list.
+  #
+  # Args:
+  #   agents: Path to the AGENTS repository (non-flake input)
+  #   languages: Optional list of language-specific rules to include
+  #   concerns: Optional list of concern rules to include
+  #             Default: [ "coding-style" "naming" "documentation" "testing" "git-workflow" "project-structure" ]
+  #   frameworks: Optional list of framework-specific rules to include
+  #
+  # Returns: A single concatenated markdown string with all selected rules.
+  #
+  # Example:
+  #   concatRulesMd {
+  #     agents = inputs.agents;
+  #     languages = [ "python" ];
+  #     concerns = [ "coding-style" ];
+  #   }
+  #   # Returns: "\n# Coding Style\n\n...python rules...\n"
+  concatRulesMd = {
+    agents,
+    languages ? [],
+    concerns ? [
+      "coding-style"
+      "naming"
+      "documentation"
+      "testing"
+      "git-workflow"
+      "project-structure"
+    ],
+    frameworks ? [],
+  }: let
+    rulePaths =
+      (map (c: {kind = "concerns"; name = c;}) concerns)
+      ++ (map (l: {kind = "languages"; name = l;}) languages)
+      ++ (map (f: {kind = "frameworks"; name = f;}) frameworks);
+
+    readRule = rule: builtins.readFile "${agents}/rules/${rule.kind}/${rule.name}.md";
+    ruleContents = map readRule rulePaths;
+  in
+    lib.concatStringsSep "\n\n" ruleContents;
+
+  # Build a coding rules section suitable for appending to AGENTS.md.
+  # Wraps concatRulesMd output with a header and HTML comment markers
+  # for idempotent updates in project-level shellHooks.
+  #
+  # Args: Same as concatRulesMd
+  #
+  # Returns: A markdown string with start/end markers and a header.
+  mkRulesMdSection = args: let
+    content = concatRulesMd args;
+  in
+    if builtins.stringLength content == 0
+    then ""
+    else ''
+      <!-- CODING-RULES:START -->
+      # Coding Rules
+
+      ${content}
+      <!-- CODING-RULES:END -->
+    '';
+
 in {
-  inherit mkCodingRules;
+  inherit mkCodingRules concatRulesMd mkRulesMdSection;
 }