feat(docs): make warm paper the default light theme

[agjs]

Summary

Make the public landing page and Astro docs light theme feel like a warm engineering notebook: cream paper and a subtle static grain, graphite serif display headings, forest-green controls, a rust headline accent and four muted inks in the architecture drawing. Shared geometry and the dark palette remain intact.

First-time visitors now start in light mode. Existing Light, Dark and Auto choices remain respected, including Auto following OS changes and persisting across reloads. Remove the nested borders around the search shortcut in both themes.

Test plan

• Docs bun run build:ci: build, generated-data checks, component checks, fragment links, rendered Markdown and agent surface
• Docs bun run a11y: zero violations across eight representative built pages
• Browser checks: first visit on a dark OS defaults to Light; saved Dark survives reload; Auto follows both OS modes and survives reload
• Desktop and 390px mobile visual review; no horizontal overflow on mobile
• Search shortcut: outer and both nested kbd elements have computed border width 0px
• CSS formatting and git diff --check

No dependency, API or application UI changes. No new remote fonts or animation dependencies. The paper texture is a small local SVG background; text remains selectable HTML.

Screenshots

Desktop:

Mobile: