scripts: check-skill-precompute-compose treats an invalid base ref as a pass because exit 2 dies inside a process substitution

[kyle-sexton]

Context

In scripts/check-skill-precompute-compose.sh:93-112, target discovery runs inside mapfile -t targets < <( case "$first" in ... esac ). The base-ref branch validates the ref and, on failure, prints an error and runs exit 2 (lines 103-106) -- but that exit terminates only the process-substitution subshell. The parent never observes its status (mapfile's own status reflects the read, and process-substitution exit codes are not propagated), so targets is simply empty and the script proceeds to scan zero files and exit 0. An invalid base ref (a typo'd branch, a shallow clone missing the ref) therefore reads as "nothing to gate": a silent pass in exactly the situation the validation was written to catch. The error text does go to stderr, but the exit code, which is what CI gates on, says success.

Proposed work

• Validate the base ref in the parent shell before the mapfile (move the git rev-parse --verify check out of the process substitution), exiting 2 from the parent on failure.
• Add a test case invoking the script with a bogus base ref and asserting exit 2 (the suite already exists for this script).

Acceptance criteria

• Invoking the script with a nonexistent base ref exits 2.
• A test covers the invalid-base-ref invocation and fails if the script exits 0.
• Valid base-ref, --all, and --paths modes behave as before.

References

• scripts/check-skill-precompute-compose.sh:93-112 (mapfile process substitution at line 93; unreachable-to-the-parent exit 2 at line 105)
• Found by the batch-simplify sweep on branch claude/code-tidying-batch-simplify-s7ljbi; deliberately not fixed there because the sweep was behavior-preserving.