fix: clarify custom provider configuration errors

[EF1874]

Summary

• document the existing providers.*.apiBase override in the root README, Agent README, and English/Chinese BYOK docs
• report the configured name for explicit unknown providers instead of provider 'null'
• preserve existing known-provider credential errors and OAuth/local/direct/Bedrock exemptions

This addresses the first two items in #128; apiType and provider registry additions remain out of scope.

Testing

• npm --prefix App/memmy-agent test -- tests/providers/providers-init.test.ts tests/providers/bedrock-provider.test.ts tests/providers/custom-provider.test.ts (21 passed)
• npm --prefix App/memmy-agent run typecheck
• built CLI validation with provider: not_registered confirmed the new error message