docs: model practice identities and appointment lifecycle #5

Build Medora's stack-neutral domain vocabulary and model from the confirmed product foundation.

Acceptance criteria:

• Create a root GLOSSARY.md containing concise domain definitions only, using the domain-modeling skill format.
• Preserve the accepted separation of patient, communication contact, communication line, practice membership, appointment request, reservation and confirmed appointment.
• Stress-test unresolved relationships using concrete scenarios; record questions, user replies, interpretation and predecessors under a new .planning session.
• Describe confirmed versus proposed domain relationships outside the glossary; do not silently decide cross-practice identity or appointment-request lifecycle details.
• Link the booking-policy and identity/permissions follow-ups without marking them resolved by vocabulary work alone.
• Select no application stack and create ADRs only if a real qualifying trade-off is resolved.

Source: confirmed product-discovery issue #3 and its scope review, merged in #4.