shell: empty-array expansion under set -u aborts on bash < 4.4 at sites that skip the house idiom

[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.

Several scripts expand possibly-empty arrays as bare "${arr[@]}" under set -u, which aborts with an unbound-variable error on bash 4.0-4.3 while the rest of the repo uses the ${arr[@]+"${arr[@]}"} guard. Worst case: plugins/work-items/scripts/backfill-capability-tier-labels.sh:44-47,62,69,135 (repo_args=() expanded while empty). Also: plugins/repo-hygiene git-tree-reset-batch.sh + git-tree-reset.sh (8 sites), scripts/check-changed-skills.sh:133, scripts/check-changelog-parity.sh:587 (+siblings).

Proposed work

• Convert the listed sites to the house ${arr[@]+"${arr[@]}"} idiom (or declare the supported-bash floor repo-wide and close as wontfix).

Acceptance criteria

• All listed sites use the guarded idiom, or the repo documents bash >= 4.4 as the floor and this class is closed.

References

• plugins/work-items/scripts/backfill-capability-tier-labels.sh:44
• plugins/repo-hygiene/skills/clean/scripts/git-tree-reset-batch.sh
• scripts/check-changed-skills.sh:133
• scripts/check-changelog-parity.sh:587

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