Add chat mention scope guard

[Entr0zy]

Summary

• Adds a focused collaborative chat mention scope guard for issue Real-time collaborative research editor & interface #12.
• Detects mentions outside section scope, blinded-reviewer leaks, external collaborators in restricted sections, notebook-cell access gaps, locked-section mention approval gaps, and private section title leakage in fanout previews.
• Includes deterministic synthetic sample data plus JSON, Markdown, and SVG reviewer artifacts.

/claim #12

Validation

• node collaborative-chat-mention-scope-guard/test.js
• node collaborative-chat-mention-scope-guard/demo.js
• git diff --check

Demo artifacts

• collaborative-chat-mention-scope-guard/reports/mention-scope-packet.json
• collaborative-chat-mention-scope-guard/reports/mention-scope-report.md
• collaborative-chat-mention-scope-guard/reports/summary.svg

Deconfliction

This gates chat/sidebar mentions before alert fanout. It is separate from discussion sidebar audit, notification visibility, private-comment export, presence privacy/liveness, suggestion provenance, clipboard import, local cache, find/replace, data availability, and section-lock arbitration slices.

Demo artifacts

• collaborative-chat-mention-scope-guard/reports/demo.webm