Summary

• add a Plugin Authoring reference doc for plugin package structure, manifests, handlers, icons, dependencies, and runtime trust boundaries
• register the doc in the Reference docs manifest
• link Plugin and Plugin Trigger node docs to the authoring guide

Validation

• cd frontend && bun run lint:check
• cd frontend && bun run typecheck
• ./check.sh (1729 tests, 1729 passed)

Review

• Code review found an expression metadata wording issue; fixed by documenting the actual runtime behavior for saved $ string config values.
• Final review reported no Critical or Important findings.