fix: correct public agent-card URLs shown to users

[Skyrider3]

The public URL surfaced to users was wrong or 404'd in several places:

• Personal card URLs are keyed by the user's handle (/personal/:handle/:slug.json), but the README intro example, the API table row, the identity-types prose, and the landing page all showed an email-based URL. The handle column is regex-constrained and can never equal an email, so following those examples 404s. Switch all of them to the handle format.
• The dashboard rendered a copyable "Public URL" for every card, even ones flagged Private or Inactive. The server only serves a card when status='active' AND is_public=TRUE, so those URLs 404. Only show the URL (and Copy button) when the card is actually served publicly; otherwise show a muted note.
• The edit-page banner gated on is_public but not status, so a public+inactive card still showed an "Open" link that 404s. Tighten it to is_public && active for consistency.

No backend, route, or schema changes.