Add tangible-ddd skill (framework's own canonical copy) (#8)

The DDD patterns skill lived in the lms-monorepo consumer; move the source
of truth into the framework repo it documents. Includes the recent guidance
additions:

- Hard Rules: a command/handler never launches a command (->send()). Feeling
  the urge signals a modeling gap upstream; reactions are inline work or an
  integration event, never a nested command. Explains the why (sync
  in-transaction publish makes ->send() a nested-tx + EventsUnitOfWork.reset()
  hazard).
- Async domain events demoted in favour of integration events: deferring a
  reaction out of the originating transaction is crossing a consistency
  boundary, so it's an integration event. AsyncWordPressActionHandler marked
  rare/last-resort with a caveat.

Co-authored-by: Titus TC <titus@teamtangible.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people