Document receiver activation flow

## Goal Document the receiver activation flow. ## Why A user receiving a workflow package should know how to inspect, import, enable, set up, and run it without understanding the internals. ## Scope - Add a receiver guide under `docs/`. - Explain package contents in plain language. - Explain safe setup and what should not be shared. - Include a minimal example using the sample resume workflow. ## Acceptance Criteria - Guide avoids private roadmap language. - Guide explains inspect-before-enable. - Guide includes commands matching the implemented CLI or clearly marks planned commands.