`git worktree add <path> <branch>` only checks out an existing ref, so
spawning an agent on a fresh feature branch failed with `fatal: invalid
reference` (exit 128) — the branch spawn asks for never exists yet, since
spawn starts from the remote's latest state.
- Detect whether the branch exists; use `git worktree add -b` to create it
when it doesn't, and a plain checkout (git DWIMs remote tracking) when it does.
- Slugify the agent name for the worktree directory so free-form names (a PR
title) no longer put spaces/colons/parens in repo-root paths.
- Surface git's stderr via a WorktreeError instead of a bare "exit status 128";
spawn re-raises it as SpawnError.
Adds tests/test_worktree.py covering new-branch creation, existing-branch
checkout, slugification, the isolation guard, and the clear-error path.
