[no-ci] Register pytest.mark authorship markers, add matching AGENTS.md instructions

[rwgk]

Summary

This adds repository-wide AGENTS.md guidance for marking the provenance of newly added unit tests using pytest markers, and registers those markers in the pytest config roots.

The marker system is intentionally minimal. The design goal is to keep agents and humans choosing from no more than three clear states:

• @pytest.mark.agent_authored(model="<model>") for tests authored by an agent and not yet materially reviewed or rewritten by a human.
• @pytest.mark.human_reviewed for agent-authored tests that a human has materially reviewed or rewritten.
• @pytest.mark.human_authored for tests authored by a human, or rewritten enough that the authorship is primarily human.

The guidance keeps markers local to the tests they describe: markers should be placed immediately above each test function, with class-level markers only when every test method in the class has the same provenance. It also says not to use module-level pytestmark for authorship provenance, because that is easy to miss in large files and makes later per-test provenance changes ambiguous.

The pytest marker registrations include sync comments so future edits keep the copies aligned across pytest config roots.

Relationship to #2232

This is an alternative to #2232. PR #2232 uses lightweight comment tags; this version uses formal pytest markers so tests can be selected by provenance with pytest -m, for example pytest -m agent_authored.