feat: add model variant support to execute plan panel

[chriswritescode-dev]

Summary

• Add variants field to ModelInfo and expose it via fetchAvailableModels
• Implement getAvailableModelVariants, getVariantDisplayLabel, and normalizeVariantForModel helpers in tui-models.ts
• Extend ExecutePlanPanel to accept and manage executionVariant / auditorVariant state, normalising selections when models load
• Rename onModelSelected callback to onSelectionChanged and include variant fields in its payload
• Pass auditorVariant through to loop runtime via runIteration
• Bump version to 0.4.6

Changes

• src/utils/tui-models.ts - ModelInfo variants field + variant helpers
• src/tui/execute-plan-panel.tsx - variant state and UI wiring
• src/loop/runtime.ts - propagate auditorVariant through runIteration
• src/utils/tui-execution-preferences.ts - variant defaults support
• src/utils/tui-client.ts - pass variant in session selection
• src/storage/migrations/131_add_loop_model_variants.sql - DB migration
• package.json - 0.4.5 → 0.4.6
• Tests added for all new utilities

Checklist

• Tests pass
• Whitespace check passes
• Reviewers assigned