fix: harden linkage JSON parse, sanitize paging args, atomic health write

- curogram server: verifyConversationPatientLink fails closed if the members
  response isn't valid JSON (e.g., an HTML proxy error) instead of throwing.
- curogram server: numArg coerces skip/take to a non-negative integer so
  floats/negatives can't break GraphQL Int validation or paging.
- supervisor: write the health file atomically (write temp + mv) so a monitor
  never reads partial JSON.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>