- Add medium/high/max thinking variants for Claude Opus 4.6 (low/medium/high/max)
- Add low/medium/high thinking variants for Claude Sonnet 4.6 Thinking
- Add 'max' to ThinkingTier type, TIER_REGEX, and THINKING_TIER_BUDGETS
- Remove antigravity-gemini-3-pro model definition
- Remove gemini-3-pro-preview CLI model definition
- Update all aliases, tests, docs, and scripts
- Include claude-sonnet-4-6-thinking model support (supersedes PR #496)
Supersedes: #496