fix: support default repo in macos issue stores

[Svector-anu]

What

Make the issue-backed cron-state and health helpers work in their documented current-repository mode on macOS Bash 3.2.

Why

Both scripts initialized REPO_ARGS=() and expanded "${REPO_ARGS[@]}" under set -u. The macOS system Bash treats that zero-element expansion as an unbound variable, so leaving GH_REPO unset aborted both helpers before gh ran:

scripts/state_store.sh: line 22: REPO_ARGS[@]: unbound variable
scripts/health_issue.sh: line 20: REPO_ARGS[@]: unbound variable

The existing state-store test even had to set a dummy GH_REPO specifically to avoid this failure. GitHub Actions uses a newer Bash, so production scheduling was not broken; the impact is local macOS operation and testing of the issue-backed self-heal path.

Fix

• Route gh issue calls through a small helper that adds --repo "$GH_REPO" only when the override is present.
• Otherwise invoke gh issue normally and let gh resolve the current repository.
• Add offline coverage with GH_REPO explicitly unset for both helpers.
• Keep the existing race/convergence assertions, while skipping the historical before-side when the resolved base already contains that older race fix.

Verification

• bash scripts/tests/test_state_store.sh — ALL PASS.
• bash scripts/tests/test_health_issue.sh — ALL PASS.
• bash -n scripts/state_store.sh scripts/health_issue.sh scripts/tests/test_state_store.sh scripts/tests/test_health_issue.sh — passed.
• ShellCheck 0.11.0, error-severity gate over all four changed shell files — passed.
• git diff --check — passed.

Mutation test on macOS Bash 3.2.57, reverting only the two source fixes while keeping the tests:

state_store mutation rc=1
scripts/state_store.sh: line 22: REPO_ARGS[@]: unbound variable
FAIL - ensure failed with GH_REPO unset (current-repo default)

health_issue mutation rc=1
scripts/health_issue.sh: line 20: REPO_ARGS[@]: unbound variable
FAIL - ensure failed with GH_REPO unset (current-repo default)

Restoring the source fixes returned both offline suites to ALL PASS.