[FEATURE] Add fableplan model alias: Fable 5 for planning, Sonnet for execution

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Feature Request

opusplan uses Opus for planning and switches to Sonnet for execution.
Now that Claude Fable 5 is available, a fableplan alias following the same
pattern would be useful for users who want Fable-level reasoning during the
planning phase with Sonnet's cost efficiency during execution.

References

• opusplan docs: https://code.claude.com/docs/en/model-config

Proposed Solution

Expected behavior

/model fableplan → Fable 5 in plan mode, Sonnet in execution mode

Current behavior

No equivalent alias exists. Users must either:

• Use opusplan (weaker planner)
• Use /model fable for the full session (expensive)

Alternative Solutions

I have tried making plan then using sonnet for execution. Works well but is a tedious process

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_