[Documentation] Add ownership indexes for new top-level folders

[ashleyshaw]

What documentation is needed?

Add README or index files explaining ownership, allowed content, and migration rules for the new top-level folders.

Parent epic: #282.

Why is this documentation important?

The restructure changes long-standing .github placement rules. Each folder needs a clear contract so future agents and maintainers do not recreate the same ambiguity.

Acceptance Criteria

• /.schemas/README.md explains schema ownership.
• /agents/README.md explains portable agent specs.
• /cookbook/README.md explains recipes and examples.
• /hooks/README.md explains safe hooks and adapters.
• /instructions/README.md explains portable instruction scope.
• /plugins/README.md explains plugin family strategy.
• /skills/README.md explains skill folder rules.
• /workflows/README.md distinguishes agentic workflows from GitHub Actions.
• Documentation is accessible and easy to find.
• PR uses correct branch prefix docs/.

[linear-code]

LS-605