feat(CF-1812): Use confluence-collab for section-based page editing

Replace inline regex section parser in voice.py with confluence_collab
library (BS4 parsing, 409 conflict retry). Bot now loads section outline
into LLM context when Confluence links are detected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

requirements.txt

@@ -11,3 +11,5 @@ openai>=2.0,<3.0
 pymupdf>=1.24,<2.0
 python-docx>=1.0,<2.0
 Pillow>=10.0,<12.0
+beautifulsoup4>=4.12
+lxml>=5.0