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

flake.lock

@@ -8,11 +8,11 @@
         ]
       },
       "locked": {
-        "lastModified": 1771401137,
-        "narHash": "sha256-RarPa2n2mTd3LhXM+cJbP9tXEUdDksqkNXVWPAQF0xw=",
+        "lastModified": 1772564781,
+        "narHash": "sha256-TQ5+ovYaNnpFl3WwT1oaj77R45BwiXnAGAIskeHhlGw=",
         "owner": "steveyegge",
         "repo": "beads",
-        "rev": "16af00d7576f8b4fd7bb59f54b344c8fbc6d7af4",
+        "rev": "cd033d5cce8362d067b994c53023b46fe2ae1f7d",
         "type": "github"
       },
       "original": {
@@ -301,11 +301,11 @@
     },
     "nixpkgs-unstable": {
       "locked": {
-        "lastModified": 1769170682,
-        "narHash": "sha256-oMmN1lVQU0F0W2k6OI3bgdzp2YOHWYUAw79qzDSjenU=",
+        "lastModified": 1772542754,
+        "narHash": "sha256-WGV2hy+VIeQsYXpsLjdr4GvHv5eECMISX1zKLTedhdg=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "c5296fdd05cfa2c187990dd909864da9658df755",
+        "rev": "8c809a146a140c5c8806f13399592dbcb1bb5dc4",
         "type": "github"
       },
       "original": {
@@ -318,11 +318,11 @@
     "perles": {
       "flake": false,
       "locked": {
-        "lastModified": 1769460725,
-        "narHash": "sha256-zM2jw+emxe8+mNyR1ebMWkQiEx8uSmhoqqI0IxXLDgs=",
+        "lastModified": 1772555208,
+        "narHash": "sha256-b1hpa7Zp6Kb6j5wo2rpI16+yfmAAyHE450lEM+XXx3s=",
         "owner": "zjrosen",
         "repo": "perles",
-        "rev": "57b20413eea461452b59e13f5a4a367953b1f768",
+        "rev": "55793dd89fe9f788d9a72fa1a7cdb0cce17c9502",
         "type": "github"
       },
       "original": {