Payment step: payment-scope the variable-resolver variables and add an option to skip the summary page

Problem/Motivation

Two small follow-ups to the orchestra_payment bridge (#3609095).

• The default resolver ("Amount from process variables") read generic variables named "amount", "currency" and "gateway". Those collide with a workflow's own variables of the same name and give no hint they drive the payment step.
• The payment step always renders a summary page ("Amount due" plus a "Go to payment" button) before redirecting to the gateway. Some flows already showed the amount upstream and just want to send the payer straight to the gateway.

Proposed resolution

• Rename the resolver's variables to "payment_amount", "payment_currency" and "payment_gateway", so they are payment-scoped and collision-free.
• Add a "Go straight to the gateway" option to the payment step. When on, a clean first arrival redirects to the gateway immediately; a failed or cancelled attempt still returns to the summary page so the payer can retry or take another outcome.

Both are covered by kernel tests. Orchestra is pre-1.0 with no released users, so the variable rename needs no upgrade path.

Remaining tasks

• Review.