Tighten agent instructions for recurring editorial issues

[danbarr]

Context

Reviews of agent-generated docs PRs (PR #1069, and the follow-up trims on #1065) keep surfacing the same editorial failure patterns: changelog-style framing ("changed from X to Y in vX.Y") in steady-state docs, negations that restate the positive statement just made, redundant note admonitions, and engineering shorthand leaking from PR descriptions into reader-facing prose. Separately, on #1065 an agent invoked via PR mention hand-edited auto-generated CLI reference files, acknowledging they'd be overwritten on the next release sync.

Gaps found

• None of the four patterns were addressed in AGENTS.md, STYLE-GUIDE.md, or the docs-review skill.
• The docs-review skill's placeholder guidance contradicted the example.com convention ([Bug]: Replace acme.com with example.com in example URLs #1070) by telling reviewers to replace example.com with real values.
• The "Auto-generated content" instruction was a soft one-liner that didn't survive a "remove this content" request, and its path list was missing the CRD pages dirs and the CRD schema barrel.
• AGENTS.md encouraged admonitions without STYLE-GUIDE.md's "don't overuse" caveat.
• The docs-review and review-docs-pr skill descriptions both claimed PR reviews (triggering ambiguity).

Fixes

• New "Document current behavior" style section in AGENTS.md and STYLE-GUIDE.md: docs are not a changelog, prefer positive statements, with a scoped exception for breaking/silently-behavioral changes as clearly labeled versioned admonitions (time-limited, prose stays standalone), plus the migration-guide pattern and deprecation-notices-are-current-state clarification.
• Four new rows in the docs-review LLM-pattern table (changelog framing, negative restatement, redundant admonitions, PR jargon leak) and corrected placeholder guidance.
• "Write the end state, not the transition" discipline in the upstream-release-docs skill.
• Rewritten "Auto-generated content" section with the why, the upstream-fix alternative, and the complete path list.