fix(scripts): check-skill-precompute-compose.sh argument parser is never exercised by its test suite

[kyle-sexton]

Found by a measured mutation run while piloting the detector-findings producer contract (#2680). Filed separately because it is a real defect in this repo's own tooling, independent of that pilot.

The gap

scripts/check-skill-precompute-compose.sh:30:

while [[ $# -gt 0 ]]; do
  case "$1" in
  --strict) STRICT=1; shift ;;
  -h | --help) usage ;;
  -- | --all | --paths)
    POSITIONAL+=("$1")
    shift
    break
    ;;
  *)
    POSITIONAL+=("$1")
    shift
    ;;
  esac
done

Mutating -gt 0 to -ge 0 survives the full suite (scripts/check-skill-precompute-compose.test.sh, baseline green, 5 tests).

It survives because every existing test enters through the -- | --all | --paths) branch, which breaks out of the loop before $# reaches 0. Nothing drains arguments through the *) fall-through, so the loop's termination condition is unasserted.

Why it matters

The script runs set -uo pipefail (line 17). With the mutation, once $# reaches 0 the loop body still executes and case "$1" hits an unbound variable. Measured behavior of the mutant, run from a real checkout:

Invocation	Original	Mutant
(no args)	usage, exit 2	line 31: $1: unbound variable, exit 1
--strict	usage, exit 2	line 31: $1: unbound variable, exit 1
<base-ref>	scans, exit 0	line 31: $1: unbound variable, exit 1
--strict <base-ref>	scans, exit 0	line 31: $1: unbound variable, exit 1
--all	scans, exit 0	scans, exit 0 (identical — break fires first)

So five of six invocation shapes break outright and the suite still reports green. The surviving mutant is not itself interesting; the oracle gap it reveals is — the argument parser has no coverage except through the one branch that short-circuits it.

Suggested fix

Add a case to scripts/check-skill-precompute-compose.test.sh that invokes the script with a bare base-ref argument (no --all, no --paths) and asserts it reaches the scan rather than erroring. A case passing --strict ahead of a base-ref would cover the *)-then-drain path as well.

Reproduction

Copy the script and its test into a scratch directory (the test resolves its subject as $(dirname "$BASH_SOURCE")/check-skill-precompute-compose.sh), apply sed -i '30s/-gt 0/-ge 0/' to the copy, and run the test — it exits 0.