In the timeline, a message group's avatar lines up with the first message bubble instead of the sender's name, so it sits one byline lower than the name it belongs to.

## Cause

In `GroupAvatar` (`packages/ui/src/components/timeline/timeline.tsx`), the avatar column gets `paddingBlockStart: BYLINE_HEIGHT + HEADER_GAP` whenever the group has a byline. That pushes the avatar below the byline and level with the first bubble. The minimum row height in `packages/ui/src/components/timeline/layout.ts` (`AVATAR_NUDGE + BYLINE_HEIGHT + HEADER_GAP + AVATAR`) depends on that offset.

## Expected

- The avatar's top lines up with the sender's byline (name, agent label, and time) for both human and agent groups. - The row height calculation in `layout.ts` matches the new position, so one-line groups don't clip the avatar and don't add extra space. - System rows that use `AVATAR_NUDGE` and `prCommentGlyphOffset` (commits, PRs, terminal) stay aligned with message rows.

(I couldn't attach a screenshot because of #91.)

Part of #5.