feat(experiments): worktree orchestrator + paired/stack/capability runners

run-experiment (git-worktree isolation, every run tracked in the ledger
whether merged or not), run-paired (length-balanced same-run head-to-head),
run-stack (skill stacking), run-capability(-indomain) (effort/model
sweeps + lift-vs-capability slope), lib (central experiments.db).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>