resemble-detect: trim body under 500 lines + add compatibility

Moves detailed request/response schemas from SKILL.md into
references/api-reference.md, bringing the SKILL body from 557 to
282 lines (validator hard cap is 500). Core decision-making content
— capability decision tree, score interpretation, workflows, red
flags — stays in the body where the agent needs it at query time.

Also adds a compatibility field to frontmatter per review feedback:
surfaces the RESEMBLE_API_KEY requirement and the public-HTTPS-URL
constraint upfront.

Author: devshahofficial