Move CONTRIBUTING + TESTING to Starlight

Status: Open — Phase 1, confirmation needed

Problem

CONTRIBUTING.md and TESTING.md are contributor-facing files at the repo root, but there is no browsable Starlight version of that guidance. The practical goal is to publish them into the future internal docs section without losing the repo-root entrypoints that GitHub and AI agents already rely on.

Steps

1. Copy CONTRIBUTING.md into docs/src/content/docs/internal/contributing.mdx (add MDX frontmatter).
2. Copy TESTING.md into docs/src/content/docs/internal/testing.mdx.
3. Keep the root files in place; they remain the repo-root entrypoints for GitHub and agent startup context.
4. Add reciprocal links so the root docs point at the browsable pages and the internal pages point back to the canonical repo files where helpful.
5. Update any repo-root docs that should advertise the new browsable pages, then verify bun run check:links:fresh passes.

What needs confirmation

• Whether the root files stay canonical or become short wrappers around the internal pages.
• Recommended: keep the root files, at least while AGENTS.md, GitHub UI, and other repo-entry docs still link to them directly.

Risk note

AGENTS.md explicitly links to both root docs today. If the source-of-truth plan changes, audit every reference first instead of assuming only TESTING.md needs updating.