Add first-class CQRS pattern guide and generated example

[JerrettDavis]

The dispatcher generator already supports command/query style request handling. Add a first-class CQRS catalog entry with real-world examples, docs, and explicit DI integration.\n\nAcceptance criteria:\n- Add a CQRS example that separates commands from queries through PatternKit's dispatcher/mediator surface.\n- Include TinyBDD coverage for command handling, query handling, pipeline behavior, and DI registration.\n- Document fluent/runtime and source-generated paths side by side.\n- Register the example through IServiceCollection.\n- Land green on CI, docs, CodeQL, and coverage.

[JerrettDavis]

Landed first-class CQRS dispatcher documentation, IServiceCollection integration, source-generated dispatcher usage, and TinyBDD coverage in PR #217.