Add claude-opus-4-6 and claude-opus-4-6[1m] to model picker

Summary

claude-opus-4-6 and claude-opus-4-6[1m] (1M context) are valid model IDs accepted by /model, but neither appears in the model picker UI. Users have to know the exact ID and type it manually.

Expected behavior

Both Opus 4.6 variants should appear as selectable options in the model picker alongside Opus 4.7, Sonnet 4.6, and Haiku 4.5.

Current behavior

Only Opus 4.7 appears as the Opus option in the picker. Setting either Opus 4.6 variant requires /model claude-opus-4-6 or /model claude-opus-4-6[1m] typed manually.