Add gitlab-list-runners.sh to enumerate GitLab runners

[benhunter]

Motivation

• Provide a small admin utility to enumerate all runners attached to a GitLab instance and offer an optional human-readable table output.

Description

• Add gitlab-list-runners.sh which invokes glab api --paginate "runners/all" and prints raw JSON by default.
• Add a --table option that requires jq and formats selected fields into TSV which is rendered with column -t for readable output, and include usage/help and argument validation.
• The script checks for the presence of glab (and jq when --table is requested) and exits with an error if missing.

Testing

• No automated tests were run for this change.

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Codex Task