feat(resilience): add connection guardian with circuit breaker and network retry

Add comprehensive connection resilience for Nia plugin tools:

- Network error retry: NiaClient retries transient errors (ECONNREFUSED,
  DNS, fetch failures) with exponential backoff up to MAX_RETRIES

- Circuit breaker: ConnectionGuardian implements CLOSED → OPEN → HALF_OPEN
  → CLOSED state machine with 30s cooldown

- Debounced reconnection: Multiple simultaneous failures trigger exactly
  ONE reconnect attempt via mutex

- Actionable errors: Credit exhaustion, rate limiting, auth errors now show
  clear guidance with URLs

- E2E tool coverage: All E2E encryption tools benefit from resilience

- Configuration: New env vars NIA_MCP_SERVER_NAME, NIA_MCP_MAX_RETRIES,
  NIA_MCP_RECONNECT_DELAY

- Tests: 453 unit tests pass, 0 failures

Fixes pre-existing lint/type issues:
- Remove unused types (Registry, TracerQueryValue)
- Fix non-null assertions in tests
- Fix 'any' types in job-manager tests
- Fix template literal patterns
- Fix AbortSignal type issues

BREAKING CHANGE: None