Pin automated review to an available Sonnet model

[sarath-menon]

The automated Claude review now authenticates with the repaired repository credential, but its default claude-sonnet-4-20250514 model returns HTTP404. This one-line change selects claude-sonnet-4-6 without changing the action version, permissions, or product code. The action documents the model input, the API lists this model, and a bounded inference request returned HTTP200 using 11 input and 5 output tokens. This draft requires explicit approval before merging: the action will issue its GitHub token only when the PR workflow matches the default branch, so adopting this configuration on main is required before the automated review on #36 can complete.

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Summary by cubic

Pins the automated Claude review to claude-sonnet-4-6 because the default claude-sonnet-4-20250514 model returns HTTP404. No other action settings, permissions, or product code change.

• This workflow must be adopted on main before the automated review can issue its GitHub token; until then the review on Count unique unfinished events for EventBus capacity #36 cannot complete.

^{Written for commit e26c471. Summary will update on new commits.}