fix(setup): measure target/ with POSIX du so macOS re-runs work

[Ash-thiago]

What and why

./setup.sh fails on macOS on every run after the first. du -sBG is GNU
syntax; BSD du rejects it with a usage error and exit 64, and under
set -euo pipefail that kills the install before anything is built.

Both the README and docs/getting-started.md document the script as
re-runnable ("Re-run ./setup.sh anytime to rebuild"), so on macOS that
instruction currently cannot be followed.

How

Swap the GNU-only du -sBG for POSIX du -sk and convert KiB to whole GiB,
rounding up so the cap keeps -BG's ceiling semantics and behaves
identically. || true routes a partial-read failure onto the existing
"could not measure target/; skipping the size cap" branch rather than
aborting the install.

-target_gb="$(du -sBG -L "${here}/target" 2>/dev/null | tail -n1 | cut -f1 | tr -d 'G')"
+target_kb="$(du -sk -L "${here}/target" 2>/dev/null | tail -n1 | cut -f1 || true)"
+if [[ "${target_kb}" =~ ^[0-9]+$ ]]; then
+  target_gb=$(( (target_kb + 1048575) / 1048576 ))
+else
+  target_gb=""
+fi

Reproduce

git clone https://github.com/WeaveMindAI/weft.git && cd weft
git checkout mvp
./setup.sh          # succeeds, creates target/
./setup.sh --cli    # fails, exit 64

$ du -sBG -L ./target
du: invalid argument to option B: G
usage: du [-Aclnx] [-H | -L | -P] [-g | -h | -k | -m] ...
$ echo $?
64

Because 2>/dev/null hides the usage error, the script prints no cause;
~/.local/share/weft/setup-runs.log records only FAIL exit=64 in section CLI.

Base branch

Targeting mvp, not main, deliberately: setup.sh does not exist on
main (v1 architecture), so this only applies here. Flagging it since recent
PRs default to main.

Linked issue

None — filing directly per CONTRIBUTING ("For small changes, just open a PR...
obvious bugs. No discussion needed.").

Type of change

• Bug fix
• New feature
• New node
• Language change (parser, type system, executor)
• Dashboard change
• Docs
• Refactor
• Other:

Checklist

• Shell-only change; bash -n setup.sh parses clean and a full
  ./setup.sh --cli --daemon now completes on macOS (it failed before).
  No Rust or frontend code touched, so cargo/pnpm gates are unaffected.
• pnpm -C dashboard check passes (if frontend changed). — n/a
• No unrelated formatting churn.
• No commented-out code.
• No TODO or FIXME without a linked issue.

Environment it was found on

macOS 26 (Darwin 25.6.0, arm64), bash 5.3.15 from Homebrew (i.e. the
documented brew install bash was done), mvp @ 0a5339f.

Anything reviewers should pay extra attention to

Two things I chose not to fold in, to keep this reviewable:

1. Rounding. GNU -BG rounds up, so I kept a ceiling rather than
   truncating. At a 60 GiB cap the difference is cosmetic, but matching the
   old behaviour exactly seemed better than quietly changing when the cap trips.

2. stat -c %Y (two call sites in the same section) is also GNU-only.
   It is not fatal thanks to the || echo 0 fallback, but on macOS both
   pre_mtime and post_mtime become 0 and therefore always compare equal,
   so the script concludes the binary did not change and skips the stale
   worker-image prune on every macOS build. Happy to fix in a separate PR if
   you want it — it felt like a different bug from this one.

[qfeuilla]

Hi, sorry about the delay I didn't see this PR. I am finishing up the compatibility and auto build of the mvp on the staging branch and I think I already did a similar fix. I will double check once I am done with the release flow