Add pi-coding-agent package, simplify Go builds with unstable.buildGoModule

- Add pi-coding-agent package (v0.55.4) with multi-platform support
- Replace custom Go 1.25.6 override with nixpkgs-unstable buildGoModule
  for beads and perles builds
- Update flake inputs: beads, nixpkgs-unstable, perles
- Update vendor hashes for beads and perles
- Add pi-coding-agent to development role

packages/beads/default.nix

@@ -2,8 +2,6 @@
 # Takes src as argument so it can be called from both overlay and flake packages
 { lib
 , buildGoModule
-, fetchurl
-, go_1_25
 , git
 , pkg-config
 , icu
@@ -11,23 +9,12 @@
 , version ? "unknown"
 }:
 
-let
-  # beads' dolt deps require Go >= 1.25.6 (GOTOOLCHAIN=auto can't download in sandbox)
-  go_1_25_6 = go_1_25.overrideAttrs (old: rec {
-    version = "1.25.6";
-    src = fetchurl {
-      url = "https://go.dev/dl/go${version}.src.tar.gz";
-      hash = "sha256-WMv3ceRNdt5vVtGeM7d9dFoeSJNAkih15GWFuXXCsFk=";
-    };
-  });
-  buildGoModule_1_25_6 = buildGoModule.override { go = go_1_25_6; };
-in
-buildGoModule_1_25_6 {
+buildGoModule {
   pname = "beads";
   inherit version src;
   subPackages = [ "cmd/bd" ];
   doCheck = false;
-  vendorHash = "sha256-M+JCxrKgUxCczYzMc2czLZ/JhdVulo7dH2YLTPrJVSc=";
+  vendorHash = "sha256-OL6QGf4xSMpEbmU+41pFdO0Rrs3H162T3pdiW9UfWR0=";
   nativeBuildInputs = [ git pkg-config ];
   buildInputs = [ icu ];
   meta = with lib; {