Clarify global agent pointer routing

[iteathen]

Purpose

Reconcile the account-global agent authority with the thin root AGENTS.md compatibility pointers already deployed across iteathen repositories.

Change

• explicitly permit a repository root AGENTS.md only as a thin compatibility/discovery pointer to the account-global authority
• make clear that the pointer is not repository authority and may not duplicate/redefine universal doctrine
• keep repository-specific agent context in AGENT_LOCAL.md
• apply the same pointer-only rule to tool/vendor instruction adapters

This is an authority-routing clarification only; it does not change repository-specific architecture or implementation policy.