fix: detect ancestor install version shadows

[aryamthecodebreaker]

Summary

• make Doctor compare an exact npm-requested package version with the running version
• fail unhealthy when a local or ancestor install overrides that request
• replace the misleading npm exec recommendation with a verified isolated-prefix/direct-shim flow
• add focused regression coverage

Verification

• CLI tests: 162 passed
• CLI typecheck and lint passed
• manual Doctor mismatch probe exits 1 with both requested and running versions

Closes #437