P13/P18 user guides

Base 630cc09, isolated wt/docs-investigation-graph-tiers. Update only the existing user pages docs/web-ui-search/investigations.md and docs/web-ui-search/pivot.md, plus this brief and a review report.

Sources: integrated backend 997cd82 (contracts/b2b-collaboration.md C9, contracts/pivot.md §12), frontend 6c8a4521 (Evidence connections, multi-pivot quote, server-tier badges). Explain saved/readable evidence, free derived view, coverage and bounded truncation, privacy, accessible controls and explicit enrichment quote/run. Explain correlation tiers, historical missing values, current favicon/probable limitations and independence from ownership or numeric scores. Do not announce P19 graph validation/reconciliation.

Preserve existing sharing/MCP/manual investigation content. Product claims must match source; use relative links and code delimiters for paths/placeholders. Finalize all Markdown before the final typecheck and full Docusaurus build, then verify generated anchors. Independent conformity by research_fix and quality by root before coordinator integration. No push.