On Linux systems, 'sg' is a mailutils command, not ast-grep. The previous
fallback would silently run the wrong binary when ast-grep wasn't found.
Changes:
- getSgCliPath() now returns string | null instead of string
- Fallback changed from 'sg' to null
- Call sites now check for null and return user-facing error with
installation instructions
- checkEnvironment() updated to handle null path
Fixes#1365
YeonGyu-Kim
7fdbabb264