feat: add the pi coding agent harness

Packages the prebuilt pi release binary and puts it on PATH alongside
Claude Code, Codex and opencode.

pi has no `@file` imports in context files, so the shared agent-skills
fragments are concatenated into ~/.pi/agent/AGENTS.md. Skills need no
wiring: pi already reads ~/.agents/skills.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

home/common/programs/dev.nix

@@ -5,6 +5,7 @@
 {
   imports = [
     ./codex.nix
+    ./pi.nix
     ./ralph-claude-code.nix
     ./t3-code.nix
   ];

home/common/programs/pi.nix

@@ -0,0 +1,32 @@
+{
+  config,
+  lib,
+  pkgs,
+  inputs,
+  ...
+}:
+let
+  pi = pkgs.callPackage ../../../pkgs/pi/package.nix { };
+
+  skills = inputs.agent-skills;
+
+  # pi has no `@file` imports in context files, so fragments get concatenated
+  # into one AGENTS.md. Skills need no wiring: pi reads ~/.agents/skills, which
+  # the agent-skills module already links.
+  agentsMd = pkgs.writeText "pi-AGENTS.md" (
+    lib.concatMapStringsSep "\n" builtins.readFile [
+      "${skills}/claude-md/machines/${config.programs.agentSkills.machine}.md"
+      "${skills}/claude-md/operating.md"
+      "${skills}/claude-md/writing.md"
+      "${skills}/claude-md/code-comments.md"
+      "${skills}/claude-md/RTK.md"
+    ]
+  );
+in
+{
+  home.packages = [ pi ];
+
+  # Settings stay unmanaged: pi writes ~/.pi/agent/settings.json itself from
+  # /settings, /trust and `pi config`.
+  home.file.".pi/agent/AGENTS.md".source = agentsMd;
+}