Feature request: fableplan model alias (Fable 5 for planning, Sonnet for execution)

Summary

Add a fableplan model alias to Claude Code, analogous to the existing opusplan alias.

Motivation

opusplan is a useful hybrid mode that uses Opus 4.6 for plan mode reasoning and automatically switches to Sonnet 4.6 for execution. Now that Claude Fable 5 is available in Claude Code (/model fable), a fableplan alias would enable the same pattern at the Fable tier:

• Plan mode → Fable 5 (maximum reasoning capability for architecture and design decisions)
• Execution mode → Sonnet 4.6 (cost-efficient implementation)

Expected behaviour

/model fableplan selects the hybrid mode. Behaviour mirrors opusplan exactly, just with Fable 5 substituted for Opus 4.6 in the planning phase.

Why this is useful

• Fable 5 is Anthropic's most capable public model and particularly strong at long autonomous reasoning — a natural fit for plan mode
• Users who want Fable-quality planning but don't need it for every code-generation step would get a significant cost saving vs running /model fable throughout
• Keeps the naming convention consistent (<model>plan) and is trivially discoverable by anyone already familiar with opusplan

References

• Existing opusplan documentation: https://support.claude.com/en/articles/11940350-claude-code-model-configuration