docs: add Cloud Code Adapter implementation plan

Comprehensive implementation plan addressing all review feedback:
- LegacyAdapter loads files only (no Parse.Cloud patching)
- triggers.js facade delegates reads AND writes with correct signatures
- AppCache persistence survives Config.put() overwrites
- _unregisterAll uses synchronous clearAll() for test cleanup
- getJobsObject() for facade backwards compatibility
- @parse-lite/cloud usage example in plan header

Author: dblythy