feat: re-add opencode-desktop as pass-through from anomalyco/opencode

Expose the opencode-desktop Electron app as a package via a pure
pass-through from a new 'opencode' flake input
(github:anomalyco/opencode/dev).

The input intentionally does NOT follow nixpkgs: upstream pins a
node_modules fixed-output derivation hash in nix/hashes.json against
its own nixpkgs snapshot, so following would risk FOD hash mismatches.

- flake.nix: add 'opencode' input (own nixpkgs, dev branch)
- pkgs/default.nix: opencode-desktop pass-through
- docs/packages/opencode.md + README.md: refreshed (Electron, not Tauri)
- CHANGELOG: unreleased entry

Verified: nix flake show lists opencode-desktop (all 4 systems);
output opencode-desktop-1.18.9+f720490 builds & wraps electron-41.

flake.lock

@@ -69,6 +69,41 @@
         "type": "github"
       }
     },
+    "nixpkgs_2": {
+      "locked": {
+        "lastModified": 1776683584,
+        "narHash": "sha256-NuTLMrr10Tng72hurYG8jYQ4XKK8wnpJmOGcPiis96g=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "9dd5558b06dbdacbf635a3dd36dce1b1a7ee3a89",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "opencode": {
+      "inputs": {
+        "nixpkgs": "nixpkgs_2"
+      },
+      "locked": {
+        "lastModified": 1785386452,
+        "narHash": "sha256-iYfkMmn1ZKbi0cv8XIAXVgPH4idzsVSK4cWyi/KGMJQ=",
+        "owner": "anomalyco",
+        "repo": "opencode",
+        "rev": "f7204902192624481e526b1852cdedaadb914e24",
+        "type": "github"
+      },
+      "original": {
+        "owner": "anomalyco",
+        "ref": "dev",
+        "repo": "opencode",
+        "type": "github"
+      }
+    },
     "openspec": {
       "inputs": {
         "nixpkgs": [
@@ -95,6 +130,7 @@
         "basecamp": "basecamp",
         "nixpkgs": "nixpkgs",
         "nixpkgs-master": "nixpkgs-master",
+        "opencode": "opencode",
         "openspec": "openspec"
       }
     }