Portless adds whole-branch worktree hostnames

Portless is solving an unglamorous but costly failure mode in parallel development: branch names that share a suffix can silently compete for the same URL.

• –`worktree.prefix: "branch"` sanitizes the full branch name, turning `feature/auth` into `feature-auth`.
• –The setting applies consistently across `run`, `get`, and zero-argument single- or multi-app modes.
• –Existing last-segment behavior remains the default, keeping upgrades backward-compatible.
• –Hostname templates and label-length handling make the feature more practical for monorepos and AI coding agents running multiple worktrees.
• –The tradeoff is slightly longer URLs, but that is a good exchange for unambiguous branch identity.