#8174: feat(wizard): strict model filtering for preferred providers

Automatically filters the model list when a provider is known (e.g. via Google Antigravity auth), reducing noise for users who only have specific provider integrations. AI Assisted; lightly tested. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adjusts the `/model` wizard’s default-model picker to more aggressively filter the model catalog when a `preferredProvider` is known, with the goal of reducing noise by narrowing the selection list up front. The implementation adds a `shouldAutoFilter` gate and tweaks provider-prompt behavior, then applies provider filtering when a preferred provider is present. This sits in `promptDefaultModel`, after allowlist filtering and before building the wizard select options. <h3>Confidence Score: 4/5</h3> - Mostly safe to merge; behavior change is localized but has a user-facing edge case around provider prompting. - Changes are confined to `promptDefaultModel` and primarily affect filtering/prompting logic. The main concern is a specific control-flow case where a non-existent preferred provider suppresses the provider filter prompt, which could regress UX for some users. No obvious runtime/type errors spotted in the diff. - src/commands/model-picker.ts (provider prompt suppression logic) <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->