[Feature]: quick-start guide for Bedrock Anthropic

Description

As a user in an enterprise that uses AWS, I would like the ability to use AWS role authentication initiate Bedrock Anthropic API calls using opencommit.

[commit] $ opencommit --provider anthropic 
Applying migration 00_use_single_api_key_and_url
│
└  Failed to apply migration 00_use_single_api_key_and_url: Error: Migration failed, set AI provider first. Run "oco config set OCO_AI_PROVIDER=<provider>", where <provider> is one of: ollama, openai, anthropic, gemini, azure, test, flowise, groq, mistral, mlx, deepseek, openrouter

Suggested Solution

1. include instructions in README on which environment variables to set to utilize Anthropic models through AWS Bedrock
2. in onboarding validation, provide bedrock-anthropic as a provider

Alternatives

Using an alternate provider - some enterprises won't allow other providers.

Additional Context

No response

Begin with the README's provider configuration instructions and the onboarding validation mentioned in the issue. Document the AWS role environment variables for Bedrock Anthropic and ensure the bedrock-anthropic provider is accepted during onboarding, then verify the example command works.