docs(recipes): add tsheadroom integration guide

[armstrongl]

Description

Contributor checklist

• Copied the integration template from templates/integration.md
• Placed submission in the correct directory under integrations/
• Filled in all required frontmatter fields
• Included working code examples
• Supplemental files (if any) are referenced from the README
• Tested the integration against a current Aperture instance

Reviewer checklist

• Template compliance: all required sections present
• Technical accuracy: hook definition, grant wiring, and response format snippets are valid and sufficient for adaptation
• Hook type categorization is correct (integration_type and additional_types if applicable)
• The dst key requirement for tailnet grants is called out
• If hook uses modify action on requests: cache impact implications are noted
• Security review: no credentials, tokens, or sensitive data in examples
• External links are valid and point to maintained resources
• CLA signed and commits are DCO signed-off

[lkosewsk]

Yes! tsheadroom is an excellent pre_request hook example! Thanks for including it!