Update the README default Claude model

[dajiaohuang]

Problem

The README lists Claude Opus 4.6 as a default model, but the current backend enum, provider configuration, model selection, and tests no longer contain it.

Solution

Replace the obsolete README model name with Claude Opus 5, which is present in the current default model-selection configuration.

Verification

• Confirmed the removed model and Opus 5 replacement in the current backend configuration and tests.
• Parsed all 135 tracked Python sources with the built-in compiler: 0 syntax errors.
• Checked 20 Markdown sources and 12 relative links: 0 unresolved local links.
• node --check passed for all four tracked JavaScript/CJS config files.
• git diff --check.

No user screenshot, account, credential, model/API, payment, telemetry, or production service was used.