Update kit docs for external work packages

[Ryzon3]

Summary

• Updates kit README and package authoring docs around Moorline as an operator-controlled event/work runtime.
• Adds shared terminology for external surfaces, external resources, work items, and package jobs.
• Documents external.event.received, onExternalEvent, package.work.manage, runtime gates, and provider.headless.run for package authors.
• Adds package-kit coverage for structurally validating an external event worker manifest.

Notes

• VISION.md is intentionally untouched because repo instructions say not to update it directly.
• This is docs/test coverage only; package-kit runtime validation already accepts capability and hook strings structurally.

Validation

• bun run check:fast
• git diff --check