docs: split MSX plugin skills by documentation scope

[Marius Storhaug (MariusStorhaug)]

Summary

• Define shared versus initiative-owned Agent Plugin marketplace boundaries.
• Canonicalize lowercase plugin IDs (msx and psmodule) and skill ownership prefixes (msx-* and psmodule-*).
• Define the flatter .github/plugin/<plugin-name>/ layout and repository-root-relative marketplace source paths.
• Split the shared msx plugin into 48 single-document pointer skills: five coding/tool standards, eleven documentation artifact types, and all thirty-two direct Ways of Working children.
• Update the plugin README and Agentic Development documentation to describe the granular skill model.
• Preserve the existing msx-* skill naming convention and shared msx plugin ownership.

Validation

• Marketplace and plugin JSON parse successfully.
• Marketplace Pester tests pass (4/4).
• Documentation index check passes.
• Documentation link validation passes (129 files scanned).
• Commit c554ac3 pushed to the PR branch.