feat: per-provider custom base URL, OpenAI Responses only (#445)

* feat: drop OpenAI chat-completions gateway format, keep Responses only

* docs: reframe custom base URL as a universal endpoint override

* docs: show optional base URL in the any-other-provider example

apps/cli/src/env.ts

@@ -30,7 +30,6 @@ import {
 const COMMON_FORWARD_VARS = [
   'SHANNON_AI_MODEL',
   'SHANNON_AI_BASE_URL',
-  'SHANNON_AI_OPENAI_FORMAT',
   // Opt-in debug flag: when set, the worker persists a bounded, sanitized snippet of a failed
   // provider turn's raw error message to error.log. Off by default; provider prose stays out of
   // durable state unless an operator deliberately enables it for a diagnosis.