fix: batch fixes for issues #2794, 2795, 2796, 2797, 2798, 2799, 2801, 2802, 2803, 2804 [skip ci] (#377)

Fixes:
- #2794: Triple backticks parsing (text_sanitize module with normalize_code_fences)
- #2795: Child process stderr interleaving (subprocess_output module)
- #2796: Working directory not restored after error (cwd_guard module)
- #2797: ASCII control characters in terminal (text_sanitize module)
- #2798: Inconsistent symbolic link resolution (path_consistency module)
- #2799: Negative duration calculations (duration_utils module with MonotonicTimer)
- #2801: File creation ignores umask (file_permissions module)
- #2802: Inconsistent case sensitivity (path_consistency module)
- #2803: Signal handler re-entrancy (signal_safety module)
- #2804: Subprocess environment sanitization (subprocess_env module)

All fixes implemented as utility modules in cortex-common crate with:
- Comprehensive documentation
- Unit tests
- Safe APIs for the identified edge cases

Co-authored-by: Bounty Bot <bounty-bot@factory.ai>
Co-authored-by: Droid Agent <droid@factory.ai>

Author: 3 people