fix: destructure stdout in Cron._readCrontab()

[avoidwork]

Description

Fixed Cron._readCrontab() which was calling .trim() on the raw { stdout, stderr } object returned by runExec(), coercing it to [object Object] instead of extracting the stdout string. This caused every method that reads the crontab (add, remove, list, enable, disable, sync) to fail silently — add() would write [object Object] into the crontab block, wiping existing managed entries.

Type of Change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor (no functional changes)
• Performance improvement
• CI / build / tooling

Testing

Updated the mock in tests/unit/scheduler/cron.test.js to return { stdout, stderr } shape matching the real runExec() return value, ensuring the test reflects actual behavior.

Coverage

• Line coverage maintained

Checklist

• npm run lint passes
• Tests pass with maintained line coverage
• No forbidden patterns used
• Conventional Commit style applied