Expose the runtime's existing branchNameOverride on `orca worktree create`
so callers can name the Git branch (skipping the configured branch prefix)
while --name keeps setting the workspace name. A valueless or empty
--branch fails before any runtime request; omitting it leaves the request
unchanged. Document the flag in help, the spec notes, and the CLI
reference, including the numeric suffix applied when the name is taken.
Fixes#19541
Co-authored-by: Neil <neil@stably.ai>