feat(worktree): add ScorpioFS backend

[Ivanbeethoven]

Summary

• add backend-neutral worktree substrate and ScorpioFS adapter
• preserve Libra ownership of Git state while ScorpioFS provides the FUSE data plane
• document lifecycle, state reconciliation, Mega single-commit push behavior, and the validated boundary
• include a regression test for clean detach with FUSE-local artifacts

Validation

• ScorpioFS backend unit tests
• worktree command integration test
• default and worktree-fuse builds
• deployed Mega project/aardvark-dns E2E: attach, status, fetch, add, commit, push --dry-run, detach