Worktrees

Note	Draft placeholder — outline notes only; prose not yet written.

Points this section needs to cover:

What a worktree is: multiple working trees sharing a single repository (one object database, one set of refs, many checkouts)
git worktree add, list, move, remove, lock/unlock, prune; git worktree add --orphan for empty starting points
Use cases:
- fixing a bug on another branch without stashing or disturbing a work in progress
- running builds/tests on one branch while working on another
- one checkout per coding agent for parallel agent work (forward pointer to Ch 7, Git and Agents)
Rules and housekeeping: a branch can only be checked out in one worktree at a time; where the linked worktree metadata lives; cleaning up with prune
New diagrams needed in diagram-source/ (shared repository, multiple working trees)