Add Claude Opus 5 support

[alejandro-ao]

Summary

• add Anthropic's claude-opus-5 to Tau's built-in direct-provider catalog
• register its 1M context window, 128k output limit, text/image input, pricing, and adaptive-thinking metadata
• map Tau xhigh to Anthropic max and send an explicit disabled-thinking request for Tau off
• document model selection and the provider compatibility decisions

User experience

Users authenticated with /login anthropic-api or /login anthropic-subscription can select Claude Opus 5 through /model or launch it with:

tau --provider anthropic -m claude-opus-5

The existing Anthropic default remains claude-sonnet-4-6.

Validation

• uv run pytest — 1,149 passed
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• hugo --source website --destination /tmp/tau-opus5-pr-site — passed with the existing taxonomy-layout warning

Sources

• https://platform.claude.com/docs/en/about-claude/models/whats-new-opus-5
• https://platform.claude.com/docs/en/about-claude/models/overview
• https://platform.claude.com/docs/en/release-notes/overview