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.

CHANGELOG.md

@@ -5,6 +5,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/).
 
 ## [Unreleased]
 
+### Added
+- `opencode-desktop` re-added as a pure pass-through from the `anomalyco/opencode` flake input (Electron app, tracked on `dev`); the input intentionally does not follow nixpkgs so upstream's `node_modules` FOD hashes stay valid
+
 ### Fixed
 - `openwork` 0.17.30: upstream renamed desktop/icon assets from `@openworkdesktop.{desktop,png}` to `openwork.{desktop,png}` and dropped the `Icon=`/`--no-sandbox` substitutions that are no longer needed