Make CLI build script Windows-friendly

[christso]

Fixes #5836

Problem

Issue 1: Build script fails on Windows The build script in packages/opencode/package.json invokes ./script/build.ts directly, relying on the shebang (#!/usr/bin/env bun) to execute. Windows doesn't honor shebangs, causing the script to fail.

Issue 2: --single flag downloads flaky baseline artifacts When building with --single (current platform only), the script still attempts to build the baseline (non-AVX2) target, which requires downloading additional Bun artifacts. These downloads frequently fail on Windows with "Failed to extract executable" errors.

Reproduction

See issue #5836 for full reproduction steps.

Changes

1. packages/opencode/package.json: Change build script from ./script/build.ts to bun run script/build.ts (Windows-compatible)
2. packages/opencode/script/build.ts: When --single is used, build only the native target by default; pass --baseline to also build the baseline (AVX2=false) variant when needed (eg. release parity)

Testing

Tested on Windows 11 with Bun 1.3.5:

bun run --cwd packages/opencode build -- --single --skip-install
# Successfully builds opencode-windows-x64 (native target only)

[rekram1-node]

/review

[github-actions[bot]]

lgtm