* feat(worktree): show checkout progress while a worktree is created
A local create spends most of its time in the checkout `git worktree add`
performs, and the card only said "Creating worktree…" for that whole time.
Git already writes its "Updating files: N%" meter into our stderr pipe, so
the add call reads it passively (same command line and environment) and the
create card shows "Checking out files… N%" with a thin bar. All six create
labels are translated so a create never switches language mid-flight.
Rebuilt on main from the version stacked on the create-gating branch. On
main a create can also claim a prepared checkout; that path never runs
`git worktree add`, so it keeps today's "Creating worktree…" label with no
bar, and a missed claim falls back to the plain add, which shows the bar.
* test(worktree): pin that finalizing a prepared checkout reports no checkout progress
Claiming a prepared checkout never runs `git worktree add`, so the create
card keeps its label with no bar; this pins that at the finalize step,
including the retarget `reset --hard`.
* test(worktree): keep only the checkout-progress tests that pin a contract
Drops unit tests that mirror the implementation; the real-git flow, the parser
captures, the cross-version Git contract and the create card test remain.
* test(worktree): pin the checkout meter's path from git to the create card
* test(worktree): merge the narrow-terminal capture's pipe chunks
