fix(skill): block transactional sends when deliverability signals are missing

[Dinu-Senal]

Transactional sends have no server-side approval gate, so a misconfigured account (sandbox domain, missing sender name, missing physical address, unverified domain) could send straight to spam with no warning.

send-transactional now runs nitro_get_status before nitro_send_message / nitro_send_test_message and stops to ask the user before proceeding if any signal is missing, instead of sending first and mentioning the risk after.
"Send anyway" only applies to the message just discussed, not to unrelated future sends in the same conversation.

I also attached what appears in terminal when user uses a misconfigured account

Testing

Manually verified via claude --plugin-dir against a deliberately misconfigured test account: confirmed the check blocks the send (not just a warning appended alongside it) and confirmed a second, different send request re-triggers the check rather than inheriting the first "send anyway."

Scope

Scoped to send-transactional only for this PR. send-campaign has the same gap (its preflight check currently runs after the test send, not before). Happy to follow up there separately if this approach looks right to you.

Open to a different pattern if you'd rather handle this server-side or via a hook instead of a skill instruction. Figured I'd propose the smallest version first.