chore(e2e): fixtures - update Opus model to Opus 5

[AndrewBarba]

Description

Update the local and Vercel e2e model matrices from Anthropic Claude Opus 4.8 to Opus 5. Keep the stable anthropic-opus check name unchanged, and update the prompt-cache fixture fallback and e2e documentation to match.

How did you test your changes?

• pnpm exec oxfmt --check on the four changed files
• git diff --check
• Confirmed no Claude Opus 4.8 references remain in the e2e workflows or fixtures
• Full e2e coverage is CI-only

PR Checklist

• I linked an issue with prior discussion confirming this change is wanted
• I ran the relevant checks from CONTRIBUTING.md
• I added tests and documentation where relevant
• I added a changeset if this touches the published eve package (not applicable; e2e/workflow-only change)
• DCO sign-off passes for every commit (git commit --signoff)

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Project	Deployment	Actions	Updated (UTC)
eve-docs	Ready	Preview, Comment, Open in v0	Jul 24, 2026 5:58pm
eve-docs-1644	Ready	Preview, Comment, Open in v0	Jul 24, 2026 5:58pm
eve-docs-4759	Ready	Preview, Comment, Open in v0	Jul 24, 2026 5:58pm