Filter /model command to only show accessible models

Summary

The /model command currently shows all supported models (including Opus 4.1, etc.) regardless of what models the user actually has access to. This can be confusing when using custom configurations like AWS Bedrock.

Current Behavior

The model selection menu displays a static list of all Claude Code supported models, even if the user cannot access some of them.

Expected Behavior

The /model command should ideally filter the list based on:

• The user's ANTHROPIC_DEFAULT_*_MODEL environment variables
• Or the models available through their configured backend (Bedrock, Vertex, direct API)

This would prevent users from selecting models that will fail with access errors.

Environment

• Using AWS Bedrock (CLAUDE_CODE_USE_BEDROCK=1)
• Custom model IDs configured via ANTHROPIC_DEFAULT_*_MODEL environment variables