fix(contracts): detect on-chain pubkey registration

[LucienSong]

Summary

• query the sender public-key registry when contract helpers choose whether to embed a key
• keep embedding for unregistered senders even when their pending nonce is greater than zero
• omit the key after canonical registration and update the recommended signing example

Rationale

Pending transaction admission no longer persists first-use keys. Nonce alone therefore cannot determine whether reference-mode signing is available when multiple transactions are queued before the first is included.

Verification

• npm test (142 passed, 1 skipped)
• npm run typecheck
• git diff --check