v10.23.0 doctor flags an empty store instead of blessing it

Added

• Empty-store detection in doctor: a store with 0 active memories for the
  resolved namespace is reported as an issue with a config hint instead of
  "healthy". A doctor run against the wrong MNEMOS_DB path or a mismatched
  MNEMOS_NAMESPACE previously passed every check vacuously (each check
  verified nothing and reported ok), which let a false all-clear survive
  during the 2026-07-05 embed-formula incident. Two hint variants: a fully
  empty database points at MNEMOS_DB (expected once for a brand-new store),
  and a database whose memories all live in other namespaces lists the
  per-namespace counts and points at MNEMOS_NAMESPACE. Populated stores gain
  a "Store populated: N active memories" check line. No auto-fix on purpose:
  an empty store has exactly two causes, fresh install (nothing to fix) or
  misconfiguration (the fix is the caller's env, and guessing where the real
  data lives is how a tool initializes an empty store at a wrong path and
  buries the actual problem). 4 new tests; 264 pass.