feat(plain-english): add standalone writing plugin

[masoudgolchin]

Summary

Adds plain-english@refact-os so users can enable a shared writing skill in Claude Code or Codex without installing Base. The skill uses common words and useful examples while preserving facts, uncertainty, and requested detail.

• Adds the pack to both marketplace catalogs, with separate host manifests and one shared skill.
• Includes install and on/off instructions, source license notices, and writing review examples.
• Releases Plain English 1.0.0 and marketplace 2.16.0. Base 1.11.1 only updates its plugin-manager catalog guidance.

Why

The reviewed Plain English draft needs its own install and on/off control. A separate pack keeps that choice independent of the rest of the toolkit while both clients use the same instructions.

Test plan

• Generated files are current; 10 packs and 44 skills validate.
• All 64 repository tests pass.
• Codex plugin validator and Claude plugin/marketplace validation pass.
• Installed the pack through each CLI in temporary configuration directories; every cached file matched the source.
• Codex's actual task input includes the enabled skill and omits it when disabled in saved user or trusted-project settings.
• Claude loaded the installed copy, completed a real writing request correctly, and accepted its enable/disable commands.

Notes

The earlier writing comparison passed all eight cases both with and without the draft. It does not establish better reader comprehension. Codex's current host check covers loading and settings; Claude's check also includes a fresh model response.

The normal user installations were not changed. Shared marketplace availability follows the merge to main.

Full test notes and limits