Generate AGENTS.md (folded into sync)

The root AGENTS.md template renders from samuel.toml:

• default_methodology — short description of the active loop
• [guardrails] — code-quality limits rendered inline
• [[plugins]] — installed skills/tools the agent can use

In v2 this body is invoked by samuel sync as part of the per-folder walk; the standalone command is a thin alias kept for backwards compatibility with v1 tooling.

The rendered output stays under 150 lines after variable expansion — that ceiling is enforced by CI on the generated output, not the template source.