fix: preserve explicit startup provider selection

src/utils/providerFlag.ts

@@ -1,3 +1,8 @@
+import {
+  clearProviderSelectionFlags,
+  EXPLICIT_PROVIDER_ENV_VAR,
+} from './providerEnvSelection.js'
+
 /**
  * --provider CLI flag support.
  *
@@ -77,6 +82,9 @@ export function applyProviderFlag(
     }
   }
 
+  clearProviderSelectionFlags()
+  process.env[EXPLICIT_PROVIDER_ENV_VAR] = provider
+
   const model = parseModelFlag(args)
 
   switch (provider as ProviderFlagName) {