docs: update all documentation to reflect apiKey refactoring (#528)

Update all documentation files that contained outdated apiKeyEnv
references to use the new apiKey approach.

## Changes

- docs/architecture.md: Update config example to use apiKey field
- docs/wf-stateless-design.md: Update config examples and type
  definitions to use apiKey instead of apiKeyEnv
- docs/builtin-agent-research.md: Update ProviderConfig type
  definition and code examples

All documentation now consistent with the code implementation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/builtin-agent-research.md

@@ -402,7 +402,7 @@ workflow 怎么配置和使用 model？
 ```136:160:packages/workflow-protocol/src/types.ts
 export type ProviderConfig = {
   baseUrl: string;
-  apiKeyEnv: string;
+  apiKey: string;
 };
 
 export type ModelConfig = {
@@ -429,7 +429,7 @@ export type WorkflowConfig = {
 export function resolveModel(config: WorkflowConfig, alias: ModelAlias): ResolvedLlmProvider {
   const modelEntry = config.models[alias];
   const providerEntry = config.providers[modelEntry.provider];
-  const apiKey = process.env[providerEntry.apiKeyEnv];
+  const apiKey = providerEntry.apiKey;
   return { baseUrl: providerEntry.baseUrl, apiKey, model: modelEntry.name };
 }
 ```