[no-ci] Add test authorship tags to AGENTS.md

[rwgk]

Summary

This adds repository-wide AGENTS.md guidance for marking the provenance of newly added unit tests with lightweight comment tags.

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

• AGENT-AUTHORED <model> for tests authored by an agent and not yet materially reviewed or rewritten by a human.
• HUMAN-REVIEWED for agent-authored tests that a human has materially reviewed or rewritten.
• HUMAN-AUTHORED for tests authored by a human, or rewritten enough that the authorship is primarily human.

The guidance keeps tags local to the tests they describe: tags should be placed immediately above each test function, with class-level tags only when every test method in the class has the same provenance. It also says to use at most one authorship tag per test and to treat missing tags as legacy or unknown provenance rather than implicit human authorship.

Relationship to #2245

An alternative pytest marker-based version is available in #2245. That version uses formal pytest.mark markers and registers them in the pytest config roots so tests can be selected by provenance. This PR keeps the simpler comment-tag approach for direct comparison.

[mdboom]

I love this concept, of course.

Rather than comments, we could use pytest.mark, which would then easily provide tools for running a subset of these tests etc.

Here's the info about adding custom markers:

https://docs.pytest.org/en/stable/example/markers.html#registering-markers

[rwgk]

Rather than comments, we could use pytest.mark, which would then easily provide tools for running a subset of these tests etc.

That's now implemented as an alternative under PR #2245.

I have a slight preference for #2245 over this PR, mostly because the decorators are formally attached.