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.

docs/packages/opencode.md

@@ -1,37 +1,56 @@
-# opencode (Deprecated)
+# opencode & opencode-desktop
 
-> **Note**: The `opencode` package has been removed from this repository.
+## opencode (CLI)
 
-## Why was it removed?
-
-OpenCode (CLI version) has been removed because there is now a well-maintained upstream repository for AI coding tools:
+The standalone `opencode` CLI is **not** packaged here. Use the well-maintained
+upstream flake instead:
 
 **[numtide/llm-agents.nix](https://github.com/numtide/llm-agents.nix)**
 
-This repository provides Nix packages for various AI coding agents, including OpenCode and others, with active maintenance and updates.
-
-## What should I use instead?
-
-Use the [llm-agents.nix](https://github.com/numtide/llm-agents.nix) flake directly:
-
 ```nix
 {
-  inputs = {
-    llm-agents.url = "github:numtide/llm-agents.nix";
-  };
-
-  outputs = { inputs, ... }: {
-    # Access packages via inputs.llm-agents.packages.${system}
-  };
+  inputs.llm-agents.url = "github:numtide/llm-agents.nix";
 }
 ```
 
-Or run directly:
-
 ```bash
 nix run github:numtide/llm-agents.nix#opencode
 ```
 
-## What about opencode-desktop?
+## opencode-desktop
 
-The `opencode-desktop` package remains available in this repository as it includes a Wayland support workaround for [upstream issue #11755](https://github.com/opencode-ai/opencode/issues/11755). Once this issue is resolved upstream, `opencode-desktop` may also be removed in favor of the llm-agents.nix repository.
+`opencode-desktop` is provided as a **pure pass-through** from the upstream
+flake input [`anomalyco/opencode`](https://github.com/anomalyco/opencode)
+(tracked on the `dev` branch). It is the official Electron desktop app built
+upstream via `nix/desktop.nix`.
+
+```bash
+nix build git+https://code.m3ta.dev/m3tam3re/nixpkgs#opencode-desktop
+nix run   git+https://code.m3ta.dev/m3tam3re/nixpkgs#opencode-desktop
+```
+
+### How it is wired
+
+- **Flake input** (`flake.nix`):
+
+  ```nix
+  opencode.url = "github:anomalyco/opencode/dev";
+  ```
+
+  It intentionally does **not** follow this repo's `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.
+
+- **Registry** (`pkgs/default.nix`):
+
+  ```nix
+  opencode-desktop = inputs.opencode.packages.${system}.opencode-desktop;
+  ```
+
+  No local derivation exists — the package is imported as-is.
+
+### Wayland
+
+The upstream wrapper already passes `--ozone-platform-hint=auto` (plus Wayland
+window decorations and IME flags) when `NIXOS_OZONE_WL` is set, so no local
+Wayland wrapper is required.