Copy isolation backends (reflink/apfs/btrfs/zfs/block-clone/rcopy)
materialise the worktree by duplicating its `.git` verbatim. When the
parent is a linked git worktree its `.git` is a pointer file, so the
isolation shared the parent's HEAD/index/ref namespace: a task's
`git checkout`/`commit` moved the parent's branch, and the rcopy
`git worktree add` path stacked task branches in the shared namespace.
`ensureIsolation` now runs `git.detachGitDir` after `isoStart`, turning
each isolation into a standalone repo with a frozen HEAD/refs/index
snapshot that borrows the source object database via
`objects/info/alternates`. Isolated git ops stay private, every task
branch is parented on the requested base, and patch/branch capture
(`git fetch <merged>`) still resolves objects.
Fixes#6003
roboomp
afea682b7f