Add Claude Opus 5 across the current model catalog and freshness checks

[jamditis]

Problem

The model guidance refreshed on July 24, 2026 still presents Claude Opus 4.8 as Anthropic's primary complex-work starting point and omits Claude Opus 5. Anthropic's current model overview lists Opus 5 and recommends it for complex agentic coding.

Because the affected pages and freshness tests label the catalog as verified on July 24, 2026, the omission makes the current-model guidance incomplete.

This was identified in the capped round-nine review of branch agent/tools-impact-burndown and is intentionally deferred from that issue-burndown PR.

Acceptance criteria

• Add Claude Opus 5 to the central model metadata and tool-comparison data.
• Update decision-tree recommendations, overview copy, downloadable comparisons, and the cost calculator where Opus 5 is the appropriate current recommendation.
• Keep Opus 4.8 only where it remains useful as an explicitly differentiated or legacy option.
• Extend freshness/consistency assertions so the catalog cannot be labeled current while omitting the provider's top complex-agent model.
• Verify claims and links against Anthropic's official model and pricing documentation at implementation time.