docs/agents.md and two persona files said /review and /test wrap the
code-reviewer and test-engineer personas. They invoke a skill in the main
session and spawn nothing (#681). The review commands also still asked for
Critical/Important/Suggestion while the skill and persona use
Critical/Required/Optional/Nit, and /ship named the old labels.
Tools that scan agents/*.md as custom agent definitions (e.g. GitHub Copilot CLI) parse every Markdown file in agents/ and require YAML frontmatter. agents/README.md was docs, not an agent, so it triggered "malformed custom agent" warnings on every session start.
Moving the docs to docs/agents.md keeps agents/ containing only real agent definitions. Updates internal links in the moved file, in the four persona files, AGENTS.md, and adds a pointer from the root README.
Fixes#258