Issue #3609120: Payment-scope the variable-resolver variables and add an option to skip the summary page

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

1. Payment-scope the variable-resolver variables. The default resolver now reads payment_amount / payment_currency / payment_gateway instead of the generic amount / currency / gateway, so they don't collide with a workflow's own variables.

2. Add a "Go straight to the gateway" option to the payment step. When on, a clean first arrival redirects to the gateway immediately, skipping the summary page; a failed or cancelled attempt still returns to the summary so the payer can retry or take another outcome.

Both are covered by new kernel tests (PaymentBridgeTest: landing shown by default, skip redirects to the gateway, resolver reads the renamed variables). phpcs/cspell clean. Orchestra is pre-1.0 with no released users, so the rename needs no upgrade path.

Closes #3609120.