fix(cron): sanitize invisible unicode in assembled prompts

[kimireg]

Summary

• Strip Unicode format-control characters from assembled cron prompts before scanning/execution
• Keep prompt-injection scanning effective by sanitizing before pattern checks
• Add regression coverage for zero-width/invisible Unicode and emoji ZWJ script output

Test plan

• python -m pytest tests/cron/test_cron_prompt_injection_skill.py
• python -m py_compile cron/scheduler.py tools/cronjob_tools.py tests/cron/test_cron_prompt_injection_skill.py
• git diff --check
• python -m pytest tests/cron

[teknium1]

This has been implemented on current main by a later maintainer fix. This is an automated hermes-sweeper review.

Evidence:

• PR fix(cron): sanitize invisible unicode in vetted skill content instead of hard-blocking #37245 merged the cron invisible-unicode fix as 2c0d64839783c98301a6ebaead5adc51e05c0cad.
• tools/cronjob_tools.py:184 defines _strip_invisible_unicode(), preserving legitimate emoji ZWJ sequences while removing invisible controls from assembled/vetted content.
• tools/cronjob_tools.py:235 applies that sanitizer before the assembled cron prompt injection scan.
• cron/scheduler.py:1280 uses the cleaned assembled prompt for cron jobs with skills or injected runtime data, so sanitized text is what proceeds to execution.
• Regression coverage in tests/tools/test_cronjob_tools.py:123 and tests/tools/test_cronjob_tools.py:143 verifies invisible Unicode is stripped without allowing hidden prompt injection through.
• The fix is contained in release tag v2026.6.5.

One intentional difference from this PR: current main keeps raw user-authored cron prompts strict-blocking on invisible Unicode, and only sanitizes vetted/runtime-loaded assembled content. That preserves the security posture while fixing the false-positive cron failure mode.