chore(compliance): retire legacy stub-based CLA system

[aabusair]

Retires the oldest of three CLA/DCO generations in this repo: a stub-distribution system where a central script wrote a cla-check-trigger.yml stub into each target repo, and that stub called a shared workflow_call workflow back here.

Archived to workflows/old/ (which disables them, since GitHub only scans .github/workflows/*.yml directly, not subdirectories):

• reusable-cla-check.yml ("Reusable — CLA Check")
• manage-cla-stubs.yml ("Org — Manage CLA Stubs (requires_cla)")

Deleted outright (nothing can invoke them once the above are archived; git history preserves them):

• scripts/cla_manager.py
• .github/scripts/check_and_install_stub.py

Confirmed dead before removing:

• reusable-cla-check.yml is on: workflow_call only, so it cannot run without a stub calling it. Org-wide code search found zero stubs in any repo — the only hits were inside this repo plus an inert test-cla-gpl2/code/placeholder.py (a .py file, never executable as a workflow).
• All 16 stub PRs the automation ever opened (head:automation/cla-stub) were confined to test-cla-gpl2 in Oct 2025, mostly closed unmerged; the same search excluding that repo returns 0. Manual PRs in Jan/Feb 2026 renamed-then-deleted the last stub out of it.
• Its only ruleset reference ("Require CLA workflow job (no license/cla)") was deleted 2026-08-16.
• check_and_install_stub.py had zero callers and its import path (.github/scripts/) no longer matches where requires_cla.py lives (scripts/), so it would fail on import. It was superseded by cla_manager.py in the 2025-10-15 refactor but never deleted.
• Reverse-dependency check confirms the four files formed a closed loop, referencing only each other.

scripts/requires_cla.py is deliberately untouched — the removed scripts imported it, but so does the live policy_selector.py engine.