ci(smoke): replace timeout with a perl alarm so the macOS job can run

[laurigates]

Closes #380. Picks up @claude's patch from the issue thread — it did the research and chose option B, but its push was rejected because the GitHub App lacks workflows permission on .github/workflows/. This session has it, so I'm landing the work.

One substantive change to that patch — see below; it had a hole that would have made the smoke test green while testing nothing.

The fix

timeout is GNU coreutils. macOS runners ship neither timeout nor gtimeout without brew install coreutils, so Build (macOS) died at command not found before sourcing anything, and has never passed since #374 added it.

Replaced with perl -e 'alarm shift; exec @ARGV; …' 60 … in both jobs, not just macOS: perl is preinstalled on both runner images, the alarm(2) timer survives the exec, and using one mechanism on both platforms means the hang-guard can't drift per-platform. That drops the coreutils assumption rather than papering over it on one side.

The correction to the proposed patch

The bot's form was perl -e 'alarm shift; exec @ARGV' 60 zsh -c '…'. perl's exec() returns only on failure, so with no tail after it the one-liner falls off the end and exits 0 when the target binary is missing:

$ timeout 60s definitely-not-a-real-binary; echo $?
timeout: failed to run command 'definitely-not-a-real-binary': No such file or directory
127

$ perl -e 'alarm shift; exec @ARGV' 60 definitely-not-a-real-binary; echo $?
0          # <- silently green

That is not hypothetical here: the Ubuntu job invokes bare zsh, installed by an earlier setup step. Had that install ever broken, this smoke test would have gone green having sourced nothing — the failure mode .claude/rules calls a lying test. timeout reported 127; the added warn + exit 127 reproduces it.

Verification

Ran the final form directly — this container has no zsh and no macOS runner, so these exercise the wrapper's semantics, which is what changed:

Case	Result	Want
success	0	0
exit 1	1	1 — real failures must still fail
exit 3	3	exit code passed through, not flattened
hang (sleep 60, alarm 2)	142 in 2s	non-zero, at the deadline — not the 6-hour job limit
blocked on stdin (cat)	142 in 2s	the actual hazard the guard exists for
missing binary	127	127 — was 0 before the correction

YAML parses and both run: blocks render as intended. actionlint isn't installed in this container, so CI is the first real lint pass — the Linters job covers it.

The issue's acceptance criterion — "a deliberately hanging ~/.zshrc still fails the job rather than running to the 6-hour limit" — is the hang and stdin rows above.

Also

Inline scripts move from double- to single-quoted zsh -c '…', dropping the \"/\$ escaping so zsh expands $ZSH_VERSION itself. Same output, less quoting to get wrong.

Once this merges, #378 gets its base merged in so its Build (macOS) re-runs against a green base — that's its last red check and the only thing standing between it and mergeable.

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