Adds delegation.worktree_isolation (default: false). When enabled, each
delegate_task child gets its own git worktree branched from the repo's
current HEAD under <repo>/.worktrees/subagent-<id>, its terminal session
starts there, and its goal message carries the isolation contract
(work + commit in the worktree; parent reviews/merges the branch).
- tools/subagent_worktree.py: clean-room implementation from Muse Code's
documented --subagent-worktree-isolation behavior (create per-child
worktree, finalize/inspect after run, auto-prune clean no-commit
worktrees, keep anything holding work).
- tools/delegate_tool.py: config gate + per-child setup in
_run_single_child; result entries gain a "worktree" field (path,
branch, commits, dirty, pruned) only when isolation engaged — the
default-off wire shape is byte-identical.
- Git-only + local-terminal-backend-only; non-git dirs, remote backends,
or any worktree failure degrade silently to shared-workspace behavior.
- Tests: tests/tools/test_subagent_worktree.py (15 tests, real git
repos) + E2E through _run_single_child with a real repo verified
parent-checkout isolation, branch reviewability, prune, and
default-off shape pinning.
- Docs: delegation feature page section + configuration.md key.
Teknium