Add claude-opus-4-8 support

[andrewlaack]

Adds Claude Opus 4.8 as a built-in model and updates the default from 4.7 to 4.8.

Changes:

• Add CLAUDE_4_8_OPUS and CLAUDE_4_8_OPUS_LONG to AnthropicModelName enum
• Add model info entries with pricing ($5/$25 per MTok, 1M context, 128k output)
• Add to _MODELS_WITHOUT_TEMPERATURE (adaptive thinking, same as 4.7)
• Add to both long-context model mappings for 1M context support
• Update DEFAULT_MODEL_ID to claude-opus-4-8
• Update .vet/models.json: rename opus alias to 4.8, keep 4.7 as opus-4.7

Tested with vet runs using both the native Anthropic connector (claude-opus-4-8) and the OpenAI-compatible connector (opus alias). Both work correctly.

[github-actions]

Vet found 0 issues.