[breaking] Anthropic retires Claude Mythos Preview in favor of Claude Mythos 5

The deprecations page states Claude Mythos Preview (claude-mythos-preview) is retired as of June 30, 2026, with a migration guide to Claude Mythos 5 (claude-mythos-5). The page does not date the announcement itself; announced_at reflects the retirement date, the only date the source provides. Effective: 2026-06-30. Migration target: anthropic/claude-mythos-5. Affected: anthropic/claude-mythos-preview. Severity: breaking. Verification: verified.