fix: add consent gate and newline sanitization to cron tool

[pgrayy]

Description

The cron tool writes LLM-supplied schedule, command, and raw entry strings directly into the user's crontab with no confirmation prompt and incomplete newline sanitization (only the description field is sanitized). This makes it possible for an agent to silently install persistent cron jobs, and for newline injection to smuggle hidden entries past any future consent prompt.

This PR adds:

• A consent gate via BYPASS_TOOL_CONSENT (matching shell, editor, file_write, and other tools) through a shared _write_crontab function that all mutating paths must use.
• Newline sanitization on composed cron lines via _sanitize_cron_line, preventing injection of hidden extra crontab entries through any input field.

Type of Change

Bug fix

Testing

• All 27 unit tests pass (hatch test tests/test_cron.py)
• Manual security validation script (save/test_cron_security.py) runs 8 tests against the real crontab, all pass
• Parametrized injection tests cover all 7 input vectors (add schedule/command/description, raw command, edit schedule/command/description)

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature
• My changes generate no new warnings
• Any dependent changes have been merged and published

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