v6: Tevrin

Spec: SPEC.md. Build log: PROGRESS.md. These files move to docs/v6/ when the version is done.

v6 has no schema changes and no new API endpoints.

• Rename. The product is now Tevrin. The CLI command is tevrin, with aide kept as a deprecated alias. Internal identifiers keep the aide name, so sessions, tokens and webhooks keep working.
• Brand and design system. A hexagon mark, an indigo to cyan gradient on slate surfaces, Geist type, and light and dark themes. See Brand.
• App refresh. A tidier shell and a Home dashboard with an onboarding checklist.
• Marketing site. Public pages, and redesigned sign-in and sign-up.
• Documentation. A short README, and this site, published to GitHub Pages.