Claude 3.5 model is no longer supported

[scottrigby]

See deprecation notice: https://platform.claude.com/docs/en/about-claude/model-deprecations#2025-08-13-claude-sonnet-3-5-models

These models were retired October 28, 2025.

We could patch the current code to use 3.7 everywhere (we already do in the Conversational workflow, but not in the Plan Execution workflow).

Note that if we wanted use Claude 4.5 we'd need to update the anthropic Go SDK. See #131 (comment)

Also note that we can't use claude 4.5 until we udpate the anthropic Go SDK, and we'll need more testing because we'd be moving from an early v0.2.0-alpha.11 to a major 1.19.0 with no backwards compatibility guarantees.