Shrink this repo's AGENTS.md to a pointer at the standard #96

Why

AGENTS.md here still carries the metadata rules (description on three surfaces, topics, keywords, classifiers, [project.urls]) that now live in the standard, section 11, plus two of the retired paragraphs (INVARIANT:, "real work not scheduled"). Two copies of the same rule drift.

Change

Replace the metadata section with one line pointing at section 11; delete the two retired paragraphs; keep what is specific to this repo (brand surfaces, the CI stale-merge-ref gotcha, the agent-skill pointers).

Done when

Nothing in AGENTS.md restates a rule the standard states.