A stray zero-width space (U+200B), BOM, or bidi control in loaded skill
markdown permanently killed any cron that loaded it. The skills-attached
assembled-prompt scan hard-blocked on any invisible-unicode char, even
though skill bodies are already install-time vetted by skills_guard.py and
the chars commonly appear in copy-pasted unicode docs / code examples.
The skills path now strips invisibles (logging the codepoints) and runs the
cleaned prompt. The raw user-prompt path (_scan_cron_prompt) keeps the hard
block — that is the actual #3968 injection surface, where a small directive
prompt with a ZWSP is a smoking gun, not prose. Stripping does not let a real
injection slip through: the directive still matches after sanitization.
_scan_cron_skill_assembled now returns (cleaned_prompt, error).
2c0d648397