ci: shared retired-repo link guard for consumer repos

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Why

Monorepo#2221 step 1 fixed the last archived reusable-workflows doc links. Step 2 needs a recorded decision on extracting the monorepo site’s retired-repo link check into a shared guard so submodule/consumer repos cannot drift again after a rename or archive.

What this issue is

Implement (or decline with reasoning) a reusable check in devantler-tech/actions that fails CI when a consumer still links a pinned retired repo URL, with an allow-list for intentional historical mentions (test fixtures, portfolio-map archival notes).

Acceptance

• Decision recorded: extract shared guard (this issue) rather than close #2221 as “docs-only forever”
• Guard ships as an opt-in composite action or reusable workflow (feature-flag-first / default-off for callers)
• Self-test covers hit + miss; monorepo#2221 can close when this lands or when explicitly declined

Part of devantler-tech/monorepo#2221

[coderabbitai]

🔗 Related PRs

#427 - fix: resolve first-party self-references at the same commit [merged]
#437 - fix(scan-for-todo-comments): restore self-checkout for post-cleanup [merged]
#451 - fix(create-issues-from-todos): re-pin todo-to-issue-action to v5.1.15 [merged]
#467 - docs: standardize opt-in default-off flag convention for shared CI [merged]
#518 - fix(create-issues-from-todos): avoid unpinned retry wrapper [merged]

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