Changelog calendar tracker and prompt list polish

The changelog page now has an interactive calendar tracker for developer activity. It maps changelog posts and committed code activity onto a monthly grid so devs can inspect what changed by date.

What changed

• docs-site/scripts/generate-activity-calendar.mjs generates static activity data from changelog markdown and committed Git stats.
• docs-site/src/components/ActivityCalendar renders the calendar, activity markers, day details, month navigation, and click-to-filter behavior.
• docs-site/src/theme/BlogListPage adds the tracker above /blog entries and filters visible changelog posts when a day is selected.
• docs-site/package.json runs activity generation before docs start and build commands, while docs-site/docusaurus.config.ts shows all posts on the blog index so filtering works across the full changelog.
• The calendar detail panel now uses container-aware layout rules to prevent hover flicker and to stack cleanly on narrower blog columns.
• css/app.css gives the Prompt list a thin neutral scrollbar, stable gutter, and removes the older duplicate prompt-list style block.

User impact

• Developers can scan changelog and code activity by date from /blog.
• Hovering a day previews the related changelogs, commits, file count, and line activity.
• Clicking a day filters the changelog list to that date, and clearing the filter restores the full list.
• The Prompt list scrollbar feels quieter and no longer shows chunky browser scrollbar buttons.

Verification

Checked locally with:

cd docs-site
pnpm activity:generate
pnpm typecheck
pnpm build

Browser checks confirmed the calendar renders on /blog, June 17 and June 18 filter to the expected changelog posts, hover previews no longer flicker, and desktop, tablet, and mobile widths have no horizontal overflow.