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.nix

@@ -16,6 +16,13 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
 
+    # opencode - open-source coding agent; provides the opencode-desktop
+    # Electron app. NOTE: intentionally does NOT follow nixpkgs. Upstream
+    # pins a node_modules fixed-output derivation hash in nix/hashes.json
+    # against its own nixpkgs snapshot; following would risk FOD hash
+    # mismatches. Tracking the `dev` default branch (flake.lock pins the rev).
+    opencode.url = "github:anomalyco/opencode/dev";
+
     # Agent definitions and coding rules
     agents = {
       url = "git+https://code.m3ta.dev/m3tam3re/AGENTS";