fix: drop OpenAI models shut down on Oct 23 and move the default off Sonnet 4.5

[volodymyrshk]

OpenAI shuts down o3-mini, o4-mini, gpt-4.1-nano and gpt-4-turbo on October 23, 2026, and Anthropic retires claude-sonnet-4-5-20250929, the default model here, on November 30 (https://developers.openai.com/api/docs/deprecations, https://platform.claude.com/docs/en/about-claude/model-deprecations).

Changes:

• Remove the four OpenAI entries from lib/models.json.
• Replace claude-sonnet-4-5-20250929 with claude-sonnet-4-6 and make it the default in app/page.tsx.

Why 4.6 and not Sonnet 5.5: the app is on ai 3.4.33, whose prepareCallSettings sends temperature: 0 when none is set, and Sonnet 5.5 rejects a non-default temperature with a 400. Sonnet 4.6 accepts it. Moving to 5.5 needs either the AI SDK upgrade or dropping temperature for that model.

Saved selections are handled by the existing effect in page.tsx (a stored model that is no longer in models.json resets to the default), and the server already rejects unknown models through resolveModel.

npm test: 76 passed. tsc --noEmit: clean.

This overlaps with #250 only in the default model; #250 now conflicts with main after #252.

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