Add a deterministic code-review diamond example

[reacher-z]

Context

The pattern package ships a real diamond constructor and runtime. A focused code-review example would make the fan-out and merge value concrete without requiring a model key.

Scope

Add a no-provider example that scopes one synthetic diff, reviews it through independent correctness and security workers, and merges structured findings.

Acceptance criteria

• Build the graph through the public diamond constructor.
• Use deterministic local handlers and portable JSON only.
• Assert observed parallelism greater than one and stable named output.
• Include one failing or rejected finding path without weakening the whole run.
• Document how real provider executors could replace handlers without embedding credentials.
• Add the example to CI and the examples index.