Fixes #93. Part of #5.

Group avatars and comment-event glyphs in the timeline now sit at the top of the row, next to the sender's byline, instead of next to the first bubble.

- `GroupAvatar` no longer adds `BYLINE_HEIGHT + HEADER_GAP` padding above the avatar. - `itemHeight` sets the row's minimum height to `AVATAR + AVATAR_NUDGE`, the same as event rows, so one-line groups don't clip the avatar or get extra space. - PR/issue comment and review rows used `prCommentGlyphOffset()` to put their glyph next to the bubble. They now use `AVATAR_NUDGE`, like every other system row (commits, PRs, terminal, presence), so they line up with message rows. `eventGlyph` returns only the icon.

Checks: `bun types` and `bun run ci` pass. I checked the lane's UI with a temporary Vite server connected to the dev host. In this channel, both the human and agent avatars start at the top of their byline (avatar top at y=181pt, byline top ≈179pt plus the 2px nudge).

### Built in Ace

Built in Ace. The visual check ran the lane's app in Safari through Ace's desktop tools. Chrome Accessibility inspection failed on a title mismatch (#113).