claude-config: fix-plugin-drift.sh --input as the last argument dies via set -u instead of the documented usage channel

[kyle-sexton]

Context

Found by the repo-wide /code-tidying:batch-simplify sweep (62 groups / 986 files, branch claude/repo-code-tidying-batch-twhddw). The sweep's doctrine files latent defects instead of fixing them mid-sweep, because the fix changes observable behavior; this issue is that filing.

plugins/claude-config/skills/audit/scripts/fix-plugin-drift.sh:54-57 — a bare --input as the final argument dies with a set -u unbound-variable error (exit 1) instead of the documented usage error (exit 2).

Proposed work

• Guard the option-argument read and route the failure through the usage channel.

Acceptance criteria

• fix-plugin-drift.sh --input (no value) exits 2 with the usage message.

References

• plugins/claude-config/skills/audit/scripts/fix-plugin-drift.sh:54-57

Filed by the /code-tidying:batch-simplify closeout (Phase 8).