Worktree preparation previously exposed only a generic fetch failure. Classify
known authentication, network, repository access, and reference-lock errors
using stable Git diagnostics, without retaining raw output or credentials.
Unknown failures keep the existing generic message.

Cover failure classification and redaction, a real missing local remote, and
propagation into a failed prepared run without creating a worktree or running
setup. The launch test waits for the persisted failure event.

Validation: 38 focused tests, server typecheck, and scoped lint passed.