fix(pstack): make worktree-audit's LAST_CHAT work with GNU stat and date

[surlymochan]

Why

worktree-audit.sh finds the newest chat transcript for each worktree with stat -f '%m %N' and turns it into a day with date -r <epoch>. Both are BSD spellings. With GNU coreutils (Linux, or macOS with Homebrew's gnubin first on PATH), stat -f prints file system status and date -r expects a file, so LAST_CHAT comes out empty and recent is always no. A worktree a chat used today then lands in review instead of verify-recent-chat, which is the bucket the cleanup playbook treats as "check before deleting".

What changed

• Probe once with stat -c %Y /. GNU gets stat -c '%Y %n' and date -d "@<epoch>"; otherwise the existing stat -f '%m %N' and date -r <epoch> stay.
• The two call sites use the probed flags. Nothing else in the script changes.

Scope

• pstack/skills/poteto-mode/scripts/worktree-audit.sh and its regenerated Claude Code copy.
• Does not touch worktree path parsing. fix(pstack): preserve worktree paths containing spaces #660 changes the same xargs stat line for paths with spaces; whichever lands second needs a one-line rebase.

Blast Radius

Only the LAST_CHAT column and the verify-recent-chat bucket of a read-only audit. On BSD stat the commands run are the same as before.

Verification

Scratch repo with one extra worktree (one commit) and a fake HOME whose agent-transcripts/chat1.json mentions that worktree path, run with -b main on Linux (coreutils 9.7):

before: 12K  0d  no  clean  no-remote  -              review              .../feat-wt
after:  12K  0d  no  clean  no-remote  -  2026-10-18  verify-recent-chat  .../feat-wt

• With a stat shim on PATH that rejects -c, the probe falls back to the BSD flags (output matches the old script, as expected on GNU), so macOS keeps its current path.
• bash -n passes, node scripts/generate-pstack-harness-trees.mjs --check reports the harness tree up to date, node scripts/check-pstack-docs.mjs reports 0 problems, and bun test in poteto-mode/scripts still passes (68/68).

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Note

Low Risk
Read-only audit script only; BSD/macOS path is unchanged when the GNU probe fails.

Overview
Fixes LAST_CHAT and the verify-recent-chat bucket on Linux (and macOS when GNU coreutils win on PATH), where BSD-only stat -f and date -r failed to read transcript mtimes.

worktree-audit.sh now probes once with stat -c %Y / and sets shared stat_fmt plus fmt_day for either GNU (stat -c '%Y %n', date -d "@epoch") or BSD (unchanged behavior). The transcript scan uses those instead of hard-coded flags.

^{Reviewed by Cursor Bugbot for commit a617232. Bugbot is set up for automated code reviews on this repo. Configure here.}