#9046: feat: Add AWS Bedrock OpenAI-compatible API provider

AWS Bedrock recently launched a new endpoint that supports OpenAI-compatible Chat Completions and Responses API. This commit adds support for this new endpoint as a model provider option during onboarding. ## Key changes - Add 'AWS Bedrock (Long Term API Key)' as a new auth choice in onboarding - Position it prominently after Anthropic in the provider list - Use OpenAI Chat Completions API implementation (openai-completions) - Support dynamic model discovery via /v1/models endpoint with 1-hour cache - Cache keyed by API key to support multiple keys in same process - Environment variable: AWS_BEDROCK_LONG_TERM_API_KEY - Endpoint: https://bedrock-mantle.us-east-1.api.aws/v1 The existing Bedrock provider (using Converse/InvokeModel API with AWS SDK credentials) remains available for customers who depend on it. The new OpenAI-compatible endpoint is recommended for new customers as it provides a simpler authentication model using Long Term API Keys. ## Files modified - src/commands/onboard-types.ts: Add bedrock-api-key AuthChoice - src/commands/auth-choice-options.ts: Add provider group and option - src/commands/auth-choice.apply.api-providers.ts: Add auth handler - src/commands/auth-choice.preferred-provider.ts: Add provider mapping - src/commands/onboard-auth.ts: Export new functions - src/commands/onboard-auth.credentials.ts: Add credential storage - src/commands/onboard-auth.config-core.ts: Add config functions - src/agents/models-config.providers.ts: Add provider builder and discovery - src/agents/model-auth.ts: Add env var mapping