fix: reverse-chronological transcript order + markdown rendering

[gbasin]

Summary

Follow-up polish to the hibernated transcript reader (patch 0.3.1), addressing feedback:

• Reverse-chronological order — newest entries render at the top, oldest at the bottom (Messages and Events). "Load earlier" moves to the bottom; older history appends below without yanking scroll, and the view opens scrolled to the top so the latest is visible.
• Markdown rendering — message bodies render as markdown via react-markdown + remark-gfm + remark-breaks (headings, bold/italic, lists, fenced code, tables, links, blockquotes). Tool output stays raw <pre>. Single newlines are preserved as line breaks.

The data model and pagination/keys are unchanged — only render order and message formatting changed.

Validation

• bun run lint && bun run typecheck && bun run test (728 pass) — updated ordering assertion + added a markdown-rendering test.
• bun run build.
• Visual check (isolated dev harness, mocked fetch): confirmed newest-at-top ordering (09:03 → 09:00 descending) and markdown rendering (heading, bold, italic, ordered/bullet lists, inline code, fenced code block, table, blockquote, link) with no literal markdown syntax left.