worktree

The worktree worker gives every agent its own isolated checkout of a shared repository. Instead of two sessions fighting over one working tree, worktree::create mints a locked git worktree on a fresh branch, records who owns it, and returns a path to use as the agent's working directory. When the work is done, worktree::land automates the merge-back: rebase onto the target branch, run an optional test command, fast-forward the target atomically, and clean the worktree up. Lands are serialized per repository through an engine FIFO queue, so parallel agents never race a merge.

The land test gate delegates to shell::exec, so the shell worker must be installed and worktree_root must sit inside its fs.host_roots jail. Landing never pushes to remotes; it moves local branches only.