fix: gate generated AGENTS guidance

[chubes4]

Summary

• make generated wp-coding-agents AGENTS.md guidance no-op unless Data Machine core's datamachine_agents_md_enabled() gate is enabled
• add regression coverage proving no sections register when the core gate is unavailable
• keep registration behavior unchanged when the core gate returns true

Testing

• bash tests/agents-md-guidance.sh

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the generated MU-plugin gate and regression coverage; Chris reviewed direction and requested the PR.