🩹 [Patch]: Route organization defaults through PSModule documentation

[MariusStorhaug]

Problem

The PSModule organization-defaults repository should keep its local agent router concise and direct readers to PSModule's central documentation through its repository context rather than duplicating cross-organization routes.

Expected outcome

• AGENTS.md lists only README.md and .github/CONTRIBUTING.md in its reading order.
• README.md describes the organization-defaults role and points to PSModule/Process-PSModule as the source of PSModule standards, agent plugins, and ways of working.
• .github/CONTRIBUTING.md is concise and points to the central PSModule contribution workflow.
• The change does not alter inherited community-default files.

Validation

Review rendered Markdown, verify changed links resolve, and confirm no unrelated files change.

[MariusStorhaug]

Completed by #80 (f424237). The change keeps AGENTS.md limited to local entry points and routes shared contribution and Process-PSModule guidance through .github/CONTRIBUTING.md, without changing inherited community-default files.