Port ripgrep auto-installation feature from original OpenCode (sst/opencode).
When ripgrep is not available, automatically downloads and installs it from
GitHub releases.
Features:
- Platform detection (darwin/linux/win32, arm64/x64)
- Archive extraction (tar.gz/zip)
- Caches binary in ~/.cache/oh-my-opencode/bin/
- New resolveGrepCliWithAutoInstall() async function
- Falls back to grep if auto-install fails
🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)
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