fix(nix): flake package should install opencode as the primary CLI

Description

The Nix flake package currently installs opencode2 as the primary CLI entrypoint and sets meta.mainProgram to opencode2.

In V2, the primary command should be opencode, while opencode2 remains available as a compatibility alias. As a result, the Nix package exposes the wrong default binary name and does not generate completions for opencode.

Plugins

N/A

OpenCode version

v2.0.2

Steps to reproduce

1. Build the package with nix build .#opencode.
2. Inspect the resulting $out/bin directory or run the packaged binary.
3. Notice that the package installs opencode2 as the primary command instead of opencode.

Screenshot and/or share link

N/A

Operating System

Nix build / Linux

Terminal

N/A