docs(adr): record the author-pair drift decision in ADR-0020

The accepted ADR still called the skill-author/agent-author duplication
an open input to #101. The branch decided it: continued drift, no sync
gate, on the measured 150-180 line overlap. State that decision so the
ADR stops contradicting the PR that closes #101, and mark the old
skill-audit citation historical.

Refs: #101

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/adr/0020-skill-description-and-body-context-contract.md

@@ -497,7 +497,12 @@ flow all remain. Cache isolation makes them structurally unavoidable
 (`skill-audit/SKILL.md:95` forbids cross-skill references; `LESSONS.md:107` records why), so the
 options are a sync gate or continued drift. This is an input to issue #101, which carries both halves
 of the kyberforge duplication problem — the deferred audit-pair merge and this — not a solved
-problem.
+problem. *Amended 2026-09-16: decided — continued drift, no sync gate. The measured overlap (about
+150–180 lines, 36 of them shared Description prose; see the rejected alternative below and ADR-0025
+point 7) does not justify a text-sync gate, so the author-pair duplication stays unguarded by
+decision. With the audit pair merged by ADR-0025, both halves of #101 are settled. The
+`skill-audit/SKILL.md:95` citation above is historical; the cross-skill reference rule now lives in
+`factory-audit`'s `references/skill-file-structure.md`, which allows only the possessive form.*
 
 **Provenance frontmatter is explicitly out of scope.** `LESSONS.md:63` asserts that non-routing
 frontmatter (`source_keys`, `category`, `version`) is loaded at agent startup, which would make the