Parallel agents get their own git worktrees

Multiple agents on one repo no longer have to fight over a single HEAD. Each worker can own a git worktree under var/worktrees/<agent-id>/.

Plan leases already decide which plan an agent owns. They do not give each agent its own checkout. One working tree means one branch tip — so parallel code edits on a shared clone thrash.

scripts/worktree_for_agent.py creates or reuses a worktree per --agent-id:

python scripts/worktree_for_agent.py ensure \
  --project /srv/myapp --agent-id mid-1 --slug fix-login
# → WORKTREE=…/var/worktrees/mid-1
# → BRANCH=feature/fix-login
# → INTEGRATION=dev

# or after a claim:
python scripts/work_once.py --once --tier mid --agent-id mid-1 --ensure-worktree

The helper ensures an integration branch (dev, else develop; if neither exists it creates dev from main/master), then checks out feature/<slug> inside that agent’s tree. Edit only under WORKTREE=. Run /commit-prep before commit; push the feature branch only — never auto-merge to dev/main.

Scaffold and project config (anchor …, --set-orchestrator) now create var/ + var/worktrees/ and append var/ to the project’s root .gitignore, so local worktrees stay untracked.

Full isolation notes: Fleet workers.