fix(scripts): validate label option values

[RerankerGuo]

Summary

Validate that --add-label and --remove-label each receive a label name before reading the next positional argument.

With set -u, invoking either option without a value previously aborted with an internal $2: unbound variable error. A following option could also be consumed as the missing value before failing later.

Changes

• Report a specific error when --add-label has no value.
• Report a specific error when --remove-label has no value.
• Reject another option token where a label name is required.

Verification

• --add-label without a value: exits 1 with a specific error and no unbound-variable message.
• --remove-label without a value: exits 1 with a specific error and no unbound-variable message.
• --add-label --remove-label bug: fails at --add-label instead of consuming the next option.
• A normal add/remove invocation still filters valid labels and calls gh issue edit with both operations.
• Ran bash -n scripts/edit-issue-labels.sh.
• Ran git diff --check.

Scope

This PR only validates option values. It does not change label existence filtering or GitHub API behavior.