feat(cli): render Markdown in agent preview

[codeaholicguy]

Summary

• render a small terminal-safe Markdown subset in agent conversation previews
• memoize parsed and laid-out message rows by messages and width so offset-only scrolling only slices visible rows
• preserve existing preview states, tailing, polling, focus, indicators, and bottom-pinned behavior
• add lifecycle documentation plus focused parser, safety, render, viewport, and memoization tests

Verification

• npm test --workspace ai-devkit -- markdownPreview.test.ts PreviewPane.test.ts computeLayout.test.ts (42 tests passed)
• npm run lint --workspace ai-devkit (0 errors; 5 pre-existing warnings)
• npm run build (6 projects passed)
• npm run test:coverage --workspace ai-devkit (984 tests passed)
• npx ai-devkit@latest lint --feature agent-console-markdown-preview
• git diff --check origin/main...HEAD

Deliberate limitations

No syntax highlighting, tables, raw HTML rendering, images, source toggle, full-history loading, semantic scroll anchors, frozen pending-update UX, or global multi-agent/multi-width cache. Unsupported and malformed input degrades to inert terminal text.