[FIX] Route Codex aliases to GPT-5.6 Sol

[Trecek]

Summary

Update AutoSkillit's canonical Codex alias registry so every local model class launches
gpt-5.6-sol. Preserve model-class behavior through reasoning effort: Sonnet-class
default work uses high, Opus-class serious work uses xhigh, and explicitly requested
Haiku-class work remains medium. Keep explicit native gpt-5.5 compatibility without
allowing any alias to select it.

• Replace the shared gpt-5.5 alias target with gpt-5.6-sol.
• Add Sol to the valid native model-ID set while retaining explicit 5.5 compatibility.
• Cover exact alias policy, legacy 5.4/mini rejection, doctor validation, native model
  passthrough, suffix stripping, and the Opus food-truck xhigh path.

Closes #4218

Implementation Plan

Plan file: /home/talon/projects/generic_automation_mcp/.autoskillit/temp/codex-loop/4218-gpt56-sol-pr-plan.md

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[Trecek]

AutoSkillit review passed. No blocking issues found.