worktrees — isolate the work before you touch it

Before you execute a plan you guarantee one thing: the work happens somewhere that can be thrown away without harming the main checkout. A feature branch keeps history clean; a worktree goes further — a second working directory on its own branch, so you can build, run, and test the feature without disturbing the files you (or another agent, or a running dev server) have open on the default branch.

worktrees is an on-demand step in the SDD chain — not a numbered phase, but the gate that implement calls when it finds itself on main. It does exactly one job: confirm an isolated workspace exists, create one if it doesn't, and hand back. It writes no runtime code and leaves no artifact under 02-DOCS; isolation is plumbing, not knowledge.

constitution → specify → clarify → plan → tasks → analyze → [ worktrees ] → implement → verify → review → ship
                                                              on-demand, called before the first commit

The one rule that defines this step