[advisory:implementation-status] Reconcile UK AI advisory ledger with 2026-05-14 GOV.UK guidance

Changed requirement IDs and source dates:

• Added UK-AI-016 (safe posture for unmaintained code) from the current GOV.UK guidance published 2026-05-14 and verified 2026-08-29.
• Added UK-AI-017 (avoid private-by-default drift) from the current GOV.UK guidance published 2026-05-14 and verified 2026-08-29.
• Updated the verification date on UK-AI-015 because the ledger changed materially in this same audit.

Summary:

• Reconciled .github/workflows/uk-ai-advisory.md and .github/workflows/advisory-uk-ai-operational-resilience.md against the current authoritative GOV.UK guidance.
• Recorded a PARTIAL gap for explicit assessment of dormant/unmaintained repository posture, including archive marking and owner/patch-route evidence for any still-live service.
• Recorded the guidance's explicit anti-private-by-default posture as a distinct stable requirement.

Notes:

• No open pull request with the [advisory:implementation-status] prefix was present at audit time.
• No separate issue was created because the ledger PR captures the single highest-priority concrete fleet improvement.

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