Add CLI system packages (DEB, RPM, macOS pkg) for amd64 and arm64

Is your feature request related to a problem?

The current primary install method for the opencode CLI is curl -fsSL https://opencode.ai/install | bash. While convenient, piping scripts to bash is risky, hard to reproduce, and difficult to manage long-term. The desktop app already ships .deb and .rpm packages via electron-builder, but the CLI has no native system packages.

Describe the solution you'd like

Add proper system packages for the CLI binary to each release:

• .deb for amd64 and arm64 (Debian/Ubuntu)
• .rpm for x86_64 and aarch64 (Fedora/RHEL/openSUSE)
• .pkg for x64 and arm64 (macOS, signed and notarized)

Packages should include the binary, shell completions, and declare ripgrep as a dependency. They should be built in the existing publish workflow and uploaded to GitHub Releases alongside the current assets.

Additional context

The curl|bash script and all existing install methods (npm, Homebrew, AUR, Nix, Docker) should remain unchanged. This adds system packages as an additional install option.

I have verified this feature doesn't already exist

• I have searched the existing issues and this feature has not been requested before