Retire the validate-naming opt-in after consumer adoption

[devantler]

🤖 Generated by the Agentic Engineer

The shared validate-naming action ships with enabled defaulting to false. Both initial consumers now explicitly opt in to v13.5.0, retain repository-owned configuration, and have removed their duplicate validators. Adoption, hosted seeded-violation RED/GREEN proof, and Platform production verification are recorded in #609.

Remaining acceptance criteria:

• Flip the default to true in a separate compatible release after refreshing both consumers' main checks, and verify omitted-input behavior.
• Remove the temporary input in a separately reviewed breaking release after checking that callers no longer require it.

Follows #609. Part of #558.