Adopt the plain-docs voice skill for the consumer docs #163

Bring over the plain-docs skill from CodoMetis.ValueRanges and CodoMetis.TypeKit as .claude/skills/plain-docs, adapted to this repository. It sets the voice for every page a consumer reads: plain, friendly, example-first and exact.

What the adaptation changes from the ValueRanges copy:

• Which pages. The root README.md, the three package READMEs under src/*/README.md, and docs/postgresql-indexes.md, docs/postgresql-constraints.md, docs/sqlserver.md and docs/upgrading.md. CONTRIBUTING.md, AGENTS.md, CLAUDE.md, ROADMAP.md, SECURITY.md and the changelogs keep their own style.
• Plain words. The table uses this library's jargon: satellite, refuse/refusal, the differ, descriptor, scaffold, seam, wiring, sentinel, whitelist, consumer.
• What must not change. The guards that exist here: DocumentationLinkTests (relative and absolute blob/main links and anchors resolve, packed READMEs link absolutely), DocumentationApiTests (every method a page names exists, and a provider page names no other provider's exclusive API), ChangelogConsistencyTests (no README carries a changelog copy, and the changelogs' Upgrading links), plus what nothing guards: code blocks are not compiled, and the headings that published changelogs and release notes link to.

The rewrite of the pages themselves is a separate issue.

Done when the skill is on main, CLAUDE.md lists it under Skills, and the unit suite passes.