Add project maintenance guide and Codex skills

[pablousx]

Problem

Project maintenance needs reusable guidance for development, debugging, releases, marketplace updates, local installation, and website changes.

Change

Add AGENTS.md with task routing, project invariants, and proportional validation, plus five repository skills with discovery metadata. Document native release runner setup, marketplace publication and updates, and companion update behavior. Link the guides from contributing documentation and clarify coordinated version changes in the publishing guide.

Validation

Passed skill frontmatter and metadata validation, local documentation link and anchor checks, shell syntax checks for command examples, repository checks, release and site static checks, and git diff --check. Changes are documentation and skill metadata only.

• Documentation is updated where needed.
• Relevant checks pass.
• The change contains no credentials or private data.