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.

pkgs/default.nix

@@ -3,7 +3,7 @@
 # Flake inputs used:
 #   inputs.basecamp  → basecamp (pass-through)
 #   inputs.openspec → openspec (pass-through)
-#   inputs.opencode → opencode-desktop (build inputs + patches)
+#   inputs.opencode → opencode-desktop (Electron desktop app, pure pass-through)
 #   inputs.agents   → not used directly here (used by lib/)
 {
   pkgs,
@@ -39,4 +39,5 @@ in {
 
   basecamp = inputs.basecamp.packages.${system}.default;
   openspec = inputs.openspec.packages.${system}.default;
+  opencode-desktop = inputs.opencode.packages.${system}.opencode-desktop;
 }