Developer Documentation and Extension System

Adapt's architecture must support pluggable modules and extensions via registration, but this is undocumented and untested end-to-end. This milestone produces living documentation that is validated by actually building and registering a working extension. Any friction or breakage encountered during that exercise is fixed before the milestone closes.

Exit criteria
Architecture and design documented for developers (contracts, module boundaries, registration pattern)
Step-by-step guide for writing a new module or extension
A reference extension implemented, registered, and tested end-to-end
Any refactoring required to make the extension work cleanly is completed
CI passes with the reference extension included