Add TrustedDealer production warning and DKG security docs

[kwsantiago]

Summary

TrustedDealer is for testing only but has no warning. Production must use distributed DKG (keep frost network dkg) where the full key never exists.

Tasks

• Add doc comment warning to TrustedDealer struct
• Add CLI warning when using keep frost generate
• Document why distributed DKG is more secure (key never exists as whole)

Key Difference

Trusted Dealer: Full key exists on one machine during ceremony
Distributed DKG: Each device contributes entropy, full key is never computed