feat(viewer): add timeline sort toggle

[wbugitlab1]

Summary

• add a viewer Timeline sort-order control with Newest First as the default and Oldest First as an option
• sort timeline observations by timestamp before filtering/pagination
• add focused VM coverage and extract the shared viewer sandbox helper

Fixes #525.

Verification

• corepack pnpm exec vitest run test/viewer-timeline-sort.test.ts test/viewer-session-id.test.ts test/viewer-memories-sort.test.ts test/viewer-i18n.test.ts
• corepack pnpm test
• corepack pnpm run build
• git diff --check
• semgrep scan --config p/default --error --metrics=off .
• osv-scanner scan source .
• gitleaks protect --staged --redact

Notes

• Viewer-only change; no REST, MCP, storage, auth, dependency, or persistence changes.
• PR target is wbugitlab1/agentmemory:main.