Both findInstallSourceRoot and findAgentsSourceRoot previously returned a
path.join(__dirname, '..', '..', '..', ...) fallback when the walk-up loop
found nothing. Replace with throw per CLAUDE.md "No Relative Path Traversal"
policy: .. chains silently break on CWD changes; a throw with the failing
__dirname in the message is an unambiguous diagnostic.
The walk-up loop already uses path.dirname iteratively (no literal ..); only
the dead-end fallback used the banned pattern.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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