docs: clarify graph extraction lifecycle on site

[wbugitlab1]

Summary

• update the website Knowledge Graph feature card to say graph extraction runs after Stop / SessionEnd
• document the real graph backfill/query routes in that card: /agentmemory/graph/build and /agentmemory/graph/query
• record Issue [upstream PR 727] docs: clarify graph extraction lifecycle on site #494 validation and verification notes under docs/todos

Fixes #494

Verification

• git diff --check
• corepack pnpm --dir website run gen-meta
• corepack pnpm --dir website exec tsc --noEmit
• corepack pnpm --dir website run build
• semgrep scan --config p/default --error --metrics=off website/components/Features.tsx docs/todos/2026-06-18-issue-494-graph-extraction-lifecycle
• gitleaks protect --staged --redact
• gitleaks detect --source . --redact --log-opts refs/remotes/origin/main..HEAD

Notes

• OSV was not run because no dependency, lockfile, container, vendored, package-manager, or third-party package surface changed.
• The broad current-tree no-git Gitleaks scan reports generated Next.js keys in ignored website/.next build output; staged and branch-diff scans are clean.