feat: add Linear webhook provider

[VeldtJumper]

Summary

• add a first-class Linear webhook provider for data-change, Issue SLA, and OAuth app revocation events
• register a canonical Linear example across the example-delivery API and Docker image
• expose Linear in the website provider grid/dropdown with branded icon attribution and setup documentation
• escape untrusted content, disable mentions, validate Linear URLs, and enforce Discord embed limits

Security note

Skyhook cannot verify Linear-Signature because generated endpoints do not retain Linear webhook signing secrets. The provider documents this limitation and treats every incoming field as untrusted display data.

Testing

• npm test — 90/90 tests passed across 32 suites
• npm run lint
• npm run build
• npm test --prefix web
• npm run build --prefix web
• Docker build and packaged Linear example runtime smoke test
• visual verification of the Linear provider tile and icon