fix(scaffold): replace non-existent extract step with select

[jackwener]

Summary

• Fix plugin-scaffold YAML template using non-existent extract pipeline step
• Replace with select: "args" which correctly extracts the args field from fetch response
• The extract step was never registered in the pipeline registry; select is the correct step for JSON path navigation

Context

opencli plugin create generates a sample hello.yaml that uses - extract: { type: json, selector: "$.args" }. This step doesn't exist in the pipeline registry (src/pipeline/registry.ts), so the generated command would fail at runtime.

Test plan

• TypeScript compilation passes
• All 7 plugin-scaffold tests pass

Part of #810 (PR C)