docs: sweep stale plan checkboxes, drop historical audit/prompts

The per-phase checkbox lists in FHIR_COMPLIANCE_PLAN.md were never
ticked off as sub-phases shipped. The "Shipped phases" table at the
bottom remains canonical; this commit just brings the per-phase lists
in sync with it.

The prompts/ and audit/ directories held driver prompts for audits and
benchmarks whose work is already in the codebase. Deleting — git
history preserves them if anyone needs to dig back.

Author: awbx