Replace Sonnet 3.7 with Sonnet 4.6 and refactor model constants to StrEnum

[hiroshinishio]

Summary

• Removed deprecated Claude Sonnet 3.7 (claude-3-7-sonnet-latest) from model chain and pricing
• Added Claude Sonnet 4.6 (claude-sonnet-4-6) at $3/$15 per MTok (input/output)
• Refactored all model constants (CLAUDE_OPUS_4_6, CLAUDE_SONNET_4_5, etc.) into a single ClaudeModelId StrEnum - no more duplicated string values
• Removed default model_id from chat_with_claude and trim_messages_to_token_limit - callers must now pass it explicitly
• Updated chat_with_agent.model_id type from str | None to ClaudeModelId | None

Social Media Post (GitAuto)

GitAuto now runs on Opus 4.6 as the primary model with Sonnet 4.6 as the first fallback. All model constants live in a single StrEnum - adding a new model is one line in one place.

Social Media Post (Wes)

Switched our primary model to Opus 4.6 and added Sonnet 4.6 as fallback. Fallback chain: Opus 4.6 -> Sonnet 4.6 -> Sonnet 4.5 -> Sonnet 4.0.