A flat .md under agents/ or commands/ whose frontmatter contains none of
the definition keys (name, description, mode, agent, model,
argument-hint, temperature, permission) is documentation, not an item:
discovery marks it Skipped with a recorded warning — it never enters a
selection, inventory, availability listing or collision index, but the
warning lands in the lockfile and add/update/select reports like other
discovery warnings. Skills keep their folder+SKILL.md marker, MCP keeps
the .yaml marker. README gains the canonical layout table and the
migration note for folder-style agent repos.
Closes beads: agent-lib-2n7
m3ta-chiron