Add four web and tool-use agent papers

[estelledc]

Summary

• add four study-v2 paper notes for the web/app/tool-use agent evaluation line: WebArena, Mind2Web, AppWorld, and ToolSandbox
• add matching study-review-receipt-v1 files with STATIC_ANALYSIS / UNVERIFIED evidence boundaries
• refresh papers atlas, note index, public count copy, and SESSION-HANDOFF

Verification

• lr search arxiv + arXiv API metadata checks for all four papers
• node scripts/quality-gate.mjs for each new note: pass, no advisories
• npm run audit:content-contract
• npm run atlas
• npm run audit:counts
• npm run audit:links
• npm run audit:wikilinks
• git ls-files -co --exclude-standard -z | node scripts/audit-public-redlines.mjs --stdin0
• npm run build:strict -- --log /tmp/study-20260715-web-tool-round-build-clean.log
• STUDY_CHANGED_FROM=origin/main npm run verify:ci

Evidence boundary

All four notes remain STATIC_ANALYSIS / UNVERIFIED. No WebArena, Mind2Web, AppWorld, or ToolSandbox benchmark environment was executed.