fix: sanitise herdr agent names to match herdr's naming contract

Fixed

• dispatchViaHerdr no longer hands herdr's agent start a name it can reject. herdr
  constrains agent names to ^[a-z][a-z0-9_-]{0,31}$, but the value passed in was an issue
  slug straight from issueSlug() — which comes from a markdown filename, so nothing stopped
  one running long or carrying an uppercase letter. New herdrAgentName() sanitises just that
  value at the herdr boundary (lowercases, strips invalid characters, forces a leading letter,
  caps at 32); the workspace's --label keeps the original, human-readable slug so a pane
  watcher still sees the real issue name.
  • registry.json: crew-afk 2.2.46 -> 2.2.47 (orchestrator/lib/dispatch.mjs).