🛡️ Sentinel: [HIGH] Fix YAML injection vulnerability in lexis frontmatter

[Jandir]

🚨 Severity: HIGH
💡 Vulnerability: Unsanitized interpolation of metadata (like video titles or filenames) into double-quoted strings within YAML frontmatter in lexis.py allowed for YAML injection or parsing breakage if the metadata contained characters like ", \n, \r, or \.
🎯 Impact: An attacker could craft a video title that breaks the structure of the generated .md/.txt volumes, potentially manipulating subsequent processing logic (like NotebookLM ingestion) or causing application crashes during YAML parsing.
🔧 Fix: Added a _escape_yaml_string function in lexis.py that safely escapes \, \n, \r, and " and applied it to variables interpolated in the _format_lexis_block output.
✅ Verification: Verified by checking that lexis.py compiles successfully and all unit tests in pytest tests/test_lexis.py and the test suite pass with no regressions.

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PR created automatically by Jules for task 9375078657468510735 started by @Jandir

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